If you plan on grinding like crazy, you'll have a large stockpile of gold.
Major Non-equipment purchases: All four Town Hall Vaults, Both Houses, The near-bottomless altar in the House of Wisdom.
Equipment Purchases: Buy a large stack of you're go-to potions (Healing, STR, etc), and a large stack of teleport scrolls.
While you can buy decent equipment, the best equipment is found in dungeons and as quest rewards. So buy a few pieced here-and-there if you need it. But some of the better stuff sold is stores can be quite expensive.