Belathor's sold all his relatives so he doesnt have any that takes over the shop. I think maybe only family members can take over the shop.

Dude speech will fix some of your problems. There is a perk that makes shopkeepers buy anything, and then there is one to increase their overall gold and you can invest to deepen that gold pool even more. Hell, I can just visit a previously useless merchant like Anoriath to unload gear if I need to.

I've invested in Speech perks before. Sometimes it's worth it, sometimes it's not. The only other alternative is to do all those missions for the TG so that you have several cities where you can unload 4,000 worth of stuf every other day. Maybe on my next playthrough I'll concentrate a little more on the Speech tree just for the hell of it. So I'll become OP and godlike at level 51 instead of level 46.

Edited to add: nevermind. I'm not waiting to get to 70 speech to invest. By the time you reach that level in speech it's likely money has become useless to you. My current character is level 52 and probably has some 50,000+ worth of merchandise stashed away that she doesn't need to sell because there's nothing she needs to buy.