We are using the same rate for buy and sell several exchanges are using that . i don't think there a problem in that.
if we are taking bit coin at 5% above market we are also giving 5% above market.
You are trading it a 5%
below market, not above. It can be sold on Bitstamp for 0.0045
BTC at the moment which would be almost 7% instant profit, it's a real cash cow you have there if you can make 5%+ on every trade

Every exchange has it own rate on black market deals you can get even more but that doesn't matter to us. If user buys a coin then also they pay BTC according to the rates we list buy order in our exchange

like if some one want to buy they can also get TNT at the same we are buying back it .