How someone can be sure you will transfer the funds after receiving BTC?
Ehh, I don't think the OP has mentioned that he'll only send his funds after receiving his coins.
I think this is a good time to point out to him and every prospective seller looking at this thread that
escrow is an option and I'd highly recommend it regardless of who you're trading with.
Here is an updated list of quality forum escrows. Trade safely.