Good question. Kayak sails are a blast when the wind cooperates. Three things I’d check before buying: how it mounts (you want something you can drop in seconds when a gust hits), whether it makes your hull pull to one side, and parts availability, since small sail makers come and go.
If the price is right and the mount is solid, go for it. Sailing back to the launch instead of paddling into the wind is pure joy.