Most Solana listings can be purchased using OpenSea, including listings created on other Solana marketplaces. If a purchase option is missing or a purchase does not go through, one of these is usually why:
A small number of listings from other marketplaces cannot be completed using OpenSea. These are shown so you can compare the market, and you can still purchase them where they were created.
Wen New Standard (WNS) items are visible and browsable, but they have no purchase, listing, or offer options.
Another marketplace may show a listing that OpenSea does not, because coverage of listings from other marketplaces differs by collection.
In the OpenSea app on Android, Solana collections are browse only for now, so no purchase, listing, or offer options appear there. Open opensea.io in your mobile browser instead.
The item may have been purchased or delisted while you were looking at it.
Compressed NFTs can be purchased, listed, and offered on. If you cannot purchase one, it is for one of the reasons above rather than the standard it uses.