IMO, the difference is purely whether you can buy things that are not possible to get otherwise and that have an impact on gameplay (so not purely cosmetic items).
Eg, if you can buy a sword that is stronger than any sword you can possibly ever get by playing the game without paying extra, then that is a P2W item.
I personally have no issue against games that allow people to advance faster than would normally be possible if they so choose to pay for it in a game that is not pvp based as long as the game is not itself designed to push people into doing that (eg the whole ‘wait 20 hours for task to complete, or pay to speed it up’)