If you fit look the first statement from the vendor as shown in the screenshot above, it clearly said "The offer you opened is a negotiation offer", which also means that before you might have accepted the trade, you must have seen it stated that the offer is a negotiation offer before accepting it. Because one thing I have noticed about people who does this kind is thing is that they usually add like 3 to 5 Naira to the current rate of dollar to Naira (i.e if the current rate is $1/#1550, they will make theirs $1/1552). So that their ads will be at the top, and the worst thing about them is that they usually don't use to sell much, as their limit is always around $5 to $10, so that if you want to buy $10 at a rate of $1/1552, they will then negotiate to deduct #100 or more from the trade. And then use it to cover the extra #2 to added to place their trade at the top. Hence, the best advice is to avoid accepting trade with any user that claims to negotiate price.