Selling 1 $100 offer for $50
1 $50 offer for $25 (spend 2 x$50 get $50 back)
So the offers to sell it are essentially $100 credit towards your phone bill for $50 or $100 towards your phone bill for $75
Why would anyone buy the $50 credit unless there are no more $100 ones
(Personally I think the $100 ones are selling for too little)