You know Warmaiden's in Whiterun? If you invest with the man in the shop, the option to invest doesn't disappear and I've gotten his buying gold up to 5000 already. It stays even if you leave dialogue with him.
Nice.

I may have to this on my non-TG Dude.
Does the trader there glitch too?
Oh, yeah.

Spoiler They'll have 11K after only one time through the dialogue.