We use wealth by level as a guideline for placing loot, but the rule with magic items is you get what you get. A character could, at any given moment, be over or undergeared for their level based on their performance and luck.
Characters, obviously, are allowed to keep all loot they acquire over the course of an adventure. Magic Items acquired this way cannot be sold for gold, nor can Magic Items be purchased for gold. The only exception to this is other PCs. You can trade magic items, or even buy and sell them between PCs. This should be done in-character in the Gameplay thread.
Additionally, random magic items will occasionally be made for sale in the Gameplay thread, where characters with sufficient gold can buy them (on a first come, first served basis). Your character can still make such purchases while still in an encounter, but they do not get access to them until they are between encounters.
Finally crafting is available and highly encouraged. Enterprising PCs could make a mint making custom magic items to order for other PCs.