Vendors becoming completely useless on some game point. They have no money to buy your stuff and they sell garbage (calling them "treasures" lol). There's a problem in fixing this also, because if they will have more money and will be able to buy lots of things from you - you will be a richman in no time: craft things, improve them, enchant them, sell. With high speechcraft And where should we spend also this gold anyway?
I think speechcraft is also pretty useless, besides maybe social perks like intimidate and persuade. Though I haven't tried investing in vendors - don't know it's one-time possibility or you can invest numerous times so will have loads of gold.