It's an equal opportunities scam the buyer has to buy the credits to pay. The seller has to pay a fee to AW when withrawing money from their AW account and have to
1. Make a minimum withdrawal of (it used to be fifty quid)
2. Wait about 2 weeks from requesting the withdrawal for the money to hit their bank account or a money order to arrive.
I suspect most AW sellers pass these costs onto the buyer which accounts for the higher prices.