The gold you see on the merchant’s side of the screen indicates the gold you have in your personal stash tab. Merchants have infinite gold, thankfully.
I’m having that problem right now. Said “can’t complete transaction” when trying to sell some gear. Ormus and Asheara both show zero gold under their tab and won’t buy anything. I had sold some, then want to Deckard to have him ID some more stuff, went back to Ormus and he has no gold and I can’t sell.
EDIT: Ormus and Asheara both say “I cannot complete that request” when trying to sell a heraldic shield. It seems that that is the only piece of gear I can’t sell. I sold a yellow Pilum, then tried the shield, no go. Then I sold a potion. Tried the shield again, and no go again. Forgot that the money under their window was my personal stash, ignore my comment on that please.
Also when trying to put the shield back in my inventory, it doesn’t go in on the first attempt, I have to try twice before it will go back in my inventory.
There are problems with selling certain items, I’ve read about rare paladin shields and rare jewels having issues sometimes (and had this bug myself a couple of times).
Today, just now, when I logged in I used a mule character and was able to sell it to Charsi in act 1. I should have tried to sell it again with the toon I originally tried, but thought about it after I already sold it. It was a rare Paladin shield, so what you have read about is what happened to me. Thanks for the reply.