Ok I had this same problem, I bought some perks speech I think I became in investor in certain merchants which means they always have more gold also don't forget fences, there is something you can do which is a little time consuming but the fences will all have 4000 gold, which is incredibly handy. Once you have completed the thieves guild quest line you can continue with the side quests such as heist jobs number jobs etc once you have done so many of these jobs in a certain area you get a big job to do, there are 4 of these once you have completed all 4 all of your fences will have 4000 gold every 2 days =)