I regard the +1 stuff that a lot of higher-level NPCs have as minor enchantments, there are thousands of such items in my campaign world. More powerful stuff (+2 & up) is considerably rarer. The 3e item costs support this - many +1 items are cheaper than nonmagical full plate, for instance. These +1 items are commonly traded for cash.
IMC PCs commonly power up their existing items, as allowed for in 3e, so once a duellist has had his rapier enchanted, say, he'll likely stick with it for the rest of his career, having further enchantments laid on it as cost and opportunity allow.
There are a few items, mostly artifacts, whose power grows along with that of their wielder, who must usually be bonded to the item in some way. These cannot be powered up normally, rather their existing powers manifest as the wielder gains in experience.