You could turn around both ebay the parts and the cars seperately. But instead of regarding modded cars as a liability, if you mod them correctly you can make more money on their initial sale. For instance:
Offroad trucks. When you add stuff to them, they actually go up in value. Tires, suspension lifts (Add-a-leafs) and nerf bars are cheap and go a long way to improve the image and usability of the truck.
For imports if you use quality parts with a real design in mind, that will make a difference. Especially when someone comes to drive it. Things like Mugen parts on a Honda never make a bad impression, Cusco suspension, exhaust pieces, etc. If they are riced out (A lot of bullshit catalog parts that make no damned difference) is when the values drop. AEM intakes, OBX exhausts, bouncy and poorly-thought-out lowering springs and pep-boys rims are what kills the resales. If you can prove the worth of your car, it WILL sell at it's value.
Once you strip the parts off, you've got nothing in resale - And it's tougher to replace those parts with stock / high quality. You'll make nothing on cars.
You could get ahold of old bikes (I get them all the time) and simple repaint them and make them look nice and turn a profit. Things like Yamaha Triples, Honda Shadows and the like will show you a return for simple work.
Lastly, look on Ebay and see whats' hot now. Shit, 1st gen Ford Broncos are worth their weight in gold. You can what to avoid (older S-10 trucks are worthless) and what to get (Suzuki Samurais are priceless in the midwest). Then you can observe trends and predict them, ready to make a sale on like, 2 free Toyota pickups and an Amigo. (Might make you $6000 in the end)
It takes work, it IS possible. I do this (But this year has SUCKED because I haven't found any free cars). I have 4 free cars right now that need to be listed. Even if I make $1000 on all 4 I've made a good profit. Not enough to quit my job, but it is enough to make a dent.