I guess they could instate a system similar to WoW had (or maybe still does) where you can "sell the item back" to the same vendor for a full refund within a short period of time if you changed your mind or made a mistake, but in most cases you can do some research on gear before purchasing.

I use Ariyala's toolkit and ffxivdb to compare slots. If I feel I'll be able to get a better piece than something I can buy with tomes, I'll wait. It's a good idea to do this if there are multiple ways to get comparable gear and then see if you're able to work towards it. Like how I don't do Savage so I can't get those pieces, but I can compare Alexander Prototype/Crafted versus Tomestone and see which is a better pick for the moment.

Soon we'll probably be able to upgrade the Shire pieces through Dun Sca...Dun...the new 24 man dungeon so I may value those pieces higher and wait to fill that slot. Planning ahead can save lots of resources...and swearing!