People have already posted about shops running out of money in the past. Here's my latest experience.

I get my current character to Leggot's for the first time. With a huge pile of equipment from two UD vaults (gnolls and then orcs) plus a ton of sis, I'm ready to cash in my stuff in a big way. I've just started to sell things when Leggot tells me he's out of money, much sooner than any shopkeeper in any previous game. So I buy a few of his minor items for a total of 371 gold pieces. No extra walking around the shop, so it couldn't have taken more than a few minutes in game time. He doesn't ring the cash box bell or give any other messages. And then I try selling more of my stuff. And he buys it... and buys more... and more! The guy ends up paying me a total of 20621 gold after he said he was broke. That's a funny way of doing business.