When I did it I got paid at the current exchange rate. They have a how it works page here:
https://btmarket.herokuapp.com/howitworksAs a vendor they make you sell the coins at $20 above the exchange rate, then you keep $15 and they keep $5.
That's a really big percentage for the vendors, given the information you provided, they take 25% of the profits you make. This is good for people who are willing to share their profits by having a platform sell their coins with ease. However, this method is very prone to scam.
I'd rather sell my own coins so I can keep all, if not most, of the profits I can get.