Board Thread:General Discussion/@comment-86.128.243.242-20150814175854/@comment-6881629-20150814192411

Merchants gold should reset after two in-game days if you buy a lot of stuff off her and go back before then she'll still have the gold you spent, otherwise my only guess is some unknown bug has stopped the reset. Her basic amount of gold without buy or selling to you (or with speech perks) should be one thousand.

Be careful though if merchants go over thirty thousand gold they usually get confused.