When you want to sell an item with a currentcy say eth, somebody can offer similar price with another currency say dai and if you do not pay attention to the offered price currency, you may have tricked with the offer like me. So opensea can pop a window saying the offered price currency is different from the selling price currency to eliminate unwanted transactions.
This is important since sometimes you do not pay attention to the currency symbols.
There are lots of bad intentioned guys in this market, opensea need to stop these tricky offerings.
Example: I have sold 0.6 eth item for 0.6 dai