I dont know about 50% value, but I seen most people selling their legit cards on purse.io.
Or just go to gyft.com
I've seen quite a few threads selling for 50-60% of the value so I'm gonna see if I can find some in that range. Not in a hurry so I'm not just gonna take every offer
If anybody is selling an AGC for 50% discount you can be sure that it was acquired by illegal means, most probably from a stolen credit card and your Amazon account can get into trouble for using that. The legit sellers who either earned it by doing work or purchased it from their cash will sell it at 10-20% discount at most.
GL finding a legit buyer who is willing to sell it for that much discount. Do ask for proof on how they acquired the gift card, no matter who you buy it from.