Sure you can buy anything you wish beyond your own level, BUT YOU CAN'T EQUIP IT unless it is AT YOUR LEVEL.So you can't really use it! So you have to sell it back for half the value you paid for the item. Credits are hard to come buy in this game. Thats the point. Cryptic game designers should fade out items above one's level to prevent this from happening
The info's all in the tooltip that pops up when you hover over the item. You should be looking at that tooltip, regardless, because it's going to tell you what the item is, what it does, etc.
Also, don't stress over vendor purchases. Vendors sell trash.