Can you clarify that 20 BTC liability to users? Is the sale including the customer's funds or not? That is, if I were to make a bid, would I owe the customers their funds in addition?
Of course these funds are not included. I explicitly stated in the first post that you have to take these obligations, however it is very unlikely that all balances will be ever claimed back.
Is the only revenue stream from ads and donations? Are there any statistics available on how much the site is making currently from those?
Currently yes, Peerbet's revenue consists only from ads and donations which are not so much (I even don't keep stats). But if you own gambling site with house edge, integrating Peerbet and its user base could significantly boost your profits.