I've never DMed, but the best DM I've ever gamed with had a unique system. He'd let you make almost any item, priced appropriately of course. If you wanted to buy a specific item, it was usually available. Stuff like relics, even the more common ones, were a luck roll(Player VS DM, high/low), with a modifier for large cities. Then he'd randomly roll loot, so we didn't usually find what we wanted in a dungeon, but might find something better. Nothing was broken, and he knew how to balance encounters to our power levels, so everything went rather well. In my experience, most of the DM's that limit items were either new to DMing, or not very good at DMing. In the right campaign, like a low-magic campaign, of course items would be restricted, but not in every campaign, and especially not to prevent someone from being "obviously OP", especially when they aren't.