FYI - at least in my state (washington), this is highly illegal.
Parting out/Selling/Reselling any used part is illegal. I've got years of experience in the auto industry in this state and seen more than a couple people get fined 1000s from the state on top of court/lawyer fees.
If you ramp this up on anything but a small scale, you will be noticed here and have to deal with the law, they do it to track stolen parts here. At least, if you plan to ramp this up, check the legalities in your state.
FYI, in my experience, wrecking yards are the big pushers for this, they invest huge into their licenses and yards, when they find out that anyone is doing what they do without a license, they go apeshit.