Even sticking within the treasure system mechanics of 4e its going to be pretty rare for a character to be able to muster up the gold required to buy an above-level item. At 1st level the PCs will find 720gp worth of treasure by the standard parcels. That's enough for a single level 3 permanent item with 40gp left over, or two level 1 items with nothing left over. The same pattern holds for all levels. The upshot being if the whole party gets together and decides that a character deserves an item and they pool all their treasure for that level, they can buy the lucky PC a level + 2 item, which is worse than all but one of the 4 items they will normally find during that same level.
In other words, theoretically its possible by RAW to buy items of ANY level if the DM lets you, but in practice you're probably mostly limited to items of right around your level. Assuming an even treasure split by the party no one single character will actually EVER have even that much cash, even if they saved every gp they ever found.