I've had four examples of what I would deem a "parts car". Two of which were <$500 dime a dozen vehicles, while the other two were vehicles that have more of an "enthusiast" following that I initially bought as "investments" where I knew I could likely get something positive from them whether I fixed it and either kept it or sold it, or parted it out.
I'll echo a lot of sentiments already expressed in this thread. Your return will largely depend on the vehicle in question and how much patience, time and space you're willing to invest.
Before spending any time pulling parts or making ads, etc., it helps to know the market for parts on the vehicle. Especially which parts, if any, are hot ticket items that would sell quickly, and/or likely bring in over $100 on it's own at some point. It's definitely not worth the time if it's something easily found in a "You-Pull-It" junkyard.
In the case of my two "dime a dozen" parts cars (a $300 elderly owned ridiculously low mile running and driving Geo Prizm that I bought for the engine, and a $400 Dodge Dakota that was absolutely destroyed inside and out with questionable brakes and suspension, but had a good V8 and trans that I eventually swapped into my V6 Dakota), I pulled the parts that I needed, then combed them for easy to store spare parts that I didn't need at the moment, parts that were new looking and/or in better shape than what was on my car, easy to adapt optional parts that the non-parts car didn't have, as well as spare change and interesting treasure left by the PO. Obviously this doesn't make you money, but will save you some money if/when something that you stored fails on the car you kept. IIRC, the Prizm had brand new tires, and the Dakota had a brand new radiator, a cool auto dimming rear view mirror and overhead console that mine didn't, some random tools in the interior and a bed half full of fire wood. I then left as much weight on them as possible (and I think threw some other junk into the back of the Dakota) and had the scrap yard haul them off and pay me the difference between the tow cost and whatever the scrap came to. On both, I would've made around 50% of what I paid to get the cars if I had a way to haul them to the scrapyard myself.
As for the other two, while I didn't initially plan to part them out, I knew before buying them that they'd have a decent return if I did go that route. One was an ultra rare (as well as rotten and abused) 1994 Mercury Capri XR2. While that car having a market might seem like a head scratcher, it ends up being a gold mine for a couple of groups. First, whether it's a base or an XR2, Festiva guys are going to be like flies on poo for the engine/trans/ecu/engine harness/etc and some of the suspension/brake parts. The other is actual Capri owners. It's a small group that tends to be more middle aged people looking to keep their cars on the road and willing to spend OK money to get parts that have been long NLA or ridiculously expensive to replace (IE a $750 IAC or $200 dist. on RockAuto). The other draw was the 1994-only parts which some of the younger Capri crowd was after to dress up their more common 91-93 Capri's. The other car was a rod knocking and abused Supercharged Cobalt SS with the "G85" package, which meant it had the Recaro seats and LSD trans. Those two parts alone ended up covering what I paid to buy the car. The other easily sold items like the SC and related parts, cylinder head, bumper covers, 18" wheels/tires and a few aftermarket parts on the car were all a bonus.
I took kind of a different approach for each car:
-On the Capri, I was pressed for time since I needed the space for another car and only had a couple weekends to mess with it. I kept the drivetrain for myself for another project and made a listing on a Capri group on Facebook to see what people wanted. I pulled the requested parts and some other pieces I kind of figured would eventually sell. I then had the scrapyard pick up what was left. I had a few more requests come in after it was gone that would've made me some more, however. Even with that and considering I kept the drivetrain, I still made more than double what I paid for the car. Most of what I intended to sell was gone within 6 months. I think I still have a small box in my garage somewhere with some little pieces that didn't sell.
-On the Cobalt, I remembered the missed opportunities with the Capri, so first I literally stripped the shell clean (other than the front and back glass) and piled absolutely everything into my garage, then took pictures of all of it and posted it on various Cobalt related groups on FB. I ended up regretting this because I was tripping over body panels, interior plastics, the sunroof assembly and the back seat, all of which were no different from what you'd find on a non-SS Cobalt littered all over junkyards. On top of that, stuff like the head/tail lights, door handles, etc. are pretty cheap new, so they got little to no interest. I would've been better off just leaving all that crap on the car and getting a few more bucks when it was scrapped. About 6 months later, I ended scrapping a lot of the bigger leftover metal parts and pitching the large non metal parts. I also steeply discounted a lot of the little misc. parts I had left at that point. I still ended up getting back just under 4 times what I spent to buy the car. I still have a decent sized bin of parts from the car in the garage that I'll probably list at some point this summer.
IMO, it's the time spent on the actual "selling" part where you'll decide how much return is worth it. I'm lucky in that as long as I pay for my non-business use, my company allows me to use their shipping accounts to get accurate costs, set up shipments and print labels. On top of that, UPS picks up packages from our office as well. Although if it's a big part, or I have several going out in one day, I'll drop it off at the UPS store on my way home so as not to be an shiny happy person to the driver. I'm not sure I'd feel the same way if I had to wait around at the UPS Store or Post Office, etc. I also have a Dad and a brother who live nearby and seem to order everything from Amazon and always have a huge supply of boxes...
It's the buyers that will drive you nuts. I guess unless you list everything on eBay, you're guaranteed to have people who:
-Will want to meet locally and get flaky by either showing up at a different time or not at all, or decide that they'll start haggling on price since you dragged whatever it is out for them
-Will "forget" about Paypal fees even if you remind them multiple times
-Message you about the availability of parts that your ad specifically notes you don't have or sold already, or asking for prices that are listed in the ad
-Ask you to quote shipping and/or take a bunch more pictures and then either don't respond, or tell you they'll buy if/when they get paid or some other factor (and usually don't end up buying)
-Ask for a shipped price for like 40lbs of stuff and then act like you're some con artist when you inform them the shipping costs more than just a few bucks
-Ask you about small insignificant pieces (down to like a singe bolt) and hope that maybe you'll just pack them up and ship them out for what it costs to ship in exchange. (That said, this can work in your favor too... I had a guy pay me $20 for the Cobalt's subframe bolts, and some lady drove from out of state to give me the $30 for Capri's dry rotted space saver...)
-Ask if a part will work on their somewhat related vehicle
-Ask if a certain part is going to fix a problem on their car (and in one case ask if I'd be willing to let them return it if it didn't....)
Even with all that, I wouldn't hesitate to do it again. I think I'd do it more like the Capri though. Pull the essentials and give people a couple weeks to let me know what they want (and will actually pay for), then scrap the rest and move on.