Like I said, I've not bought from any place that has a BP and I've never sold anything, but it seems like everything is based on the hammer price, no?
The seller is charged a fee (eg, 15%) based on the hammer price ($1000 - $150 --> $850).
The buyer is charged a fee (eg, 12%) based on the hammer price ($1000 + $120 --> $1120).
If this is correct, I could see where someone unfamiliar with how it works could get 'burned' the first time, thinking they would get 85% of $1150. But as long as the terms are clear, then it should be fairly straight-forward, I would think.