I'm guessing you got screwed by a wide bid/ask spread. Basically, every stock has bids (offers to buy) and asks (offers to sell) at various prices. The bids are always below the asks (because if they cross over a trade happens and those offers go away). Usually, the difference is so small that we talk about a single price, but in thinly trades stocks it can be significant.

Next time use a limit order (or just don't trade penny stocks).