When trading early in FIFA 21 you have to be careful when referencing websites such as FUTBin and FUTWiz etc. In the first few days of every FIFA prices fluctuate and the tracking of the sales seems to vary between sites. Potentially meaning that selling prices shown on the sites can be wrong.
To make sure you don’t get caught out research players that you are attempting to trade in. Check a few websites and also the transfer market to see what players are selling for in open bids. Below shows screenshots of Neymar at the same time on each site. One has up to date price on PS4 and one on Xbox. Make sure you check!