Because they are obtained illegally either from the person who is selling them or from the person the seller got them from.
FYI AGC can be reversed up to a week later, Amazon will email you that your account is closed due to GC problem..
Also auto is not quite a good idea as u get agc codes after u paid the money & the agc codes can be invaild..
OP why not u try escrow then u will have more buyers.. I will be one of them too..
Yes it is middleman, find a good rep middleman here & ask the buyers to pay fees. You dont lose extra but gain more trust.
Send code to buyer after btc is with escrow, ask buyer to redeem agc & give proof if it is not vaild.