Vendors should restock random merchandise every level. Unique and pre-set items will remain until they are purchased, and gold does not regenerate.
If you tend to use more found equipment than purchased, and sell the loot you are not going to use, you should be able to accumulate enough gold to learn all skill upgrades and respec a couple of times. Playing a ranger I had no issue with running out of gold, purchasing a few equipment upgrades (especially earlier in the game, and in DKS the unique bows will pretty much always be better than anything else you can find, and I bought some equipment just to get the enchantment formula) and respecing once.
If other merchants have gold and you don't have a lot of loot to sell, head to a small level (the harbour building, or a house in High Hall, etc) and have your runners go out repeatedly for malachite ore and diamonds to sell, as well as whatever herbs you need for the best healing and rejuvenation potions you can make.Runners and Tower ?
/ Very low success rate with Fanny Blossom?