The most important thing you should do when you play FF12 is research their ty bazaar stuff. If you want two items that take 6 of item B each, then it makes sense that you should be able to just sell 12 of those items. But SE are cwaaaazy, so if you sell 12 and buy the item, the amount of B you have sold to the bazaar is reset to 0. You could sell 99 of them at once and still only get one bazaar good. But if you sold them 6 at a time you get as many as you want. It's stupid.

However, the bazaar only matters if you want to get the super awesome bestest weapons. If not, then just sell whatever. If you do want the weapons, then you'd better get some research in because there is no chance you're gonna be able to fluke these items. It's insane. Things to definitely hold onto are souls. Any soul, just keep it. I know of a few others you should keep until you start going for the uber weapons, but can't remember the names unfortunately.

If this is your first playthrough, don't worry about the bazaar. In fact, unless you're planning on keeping the game and getting the guide, then you can just sell to the bazaar and not bother with uber weapons. Though it is, of course, up to you. The internet has plenty of guides and info on getting the most out of the bazaar.