I guess the merchant you traded with was using Floating rate. There are two types of rate when placing an add on any exchange 'Floating rate and Fixed rate' and you were supposed to see a pop message before placing the order which give you the window to decide either to continue with the merchant or not. I have experienced this on a good number of exchanges and I don't see it as scam. It's a good thing you shared cos it has provided an opportunity to enlighten users further.
Do you have a Bitget account and have tried crypto/naira P2P there? I have used over 10 exchanges just for P2P before. Go to Bitget and understand what I am talking about than to disagree with my what I said about Bitget. The exchange is full of scamming ads as a result of poor rate algorithm the exchange is using. I have seen people that used floating rate on other exchanges like Binance, Bitget etc cetera. If the price changes, the exchange will take you to the exchange P2P page to select another offer. You still have a lot to learn.