UO Second Age

NPC vendors in the T2A era didn't have unlimited amounts of money to buy items from players. I remember that things like selling gems to the gem dealer in Brit was nearly impossible because people were constantly selling there so the vendor had no money to buy with.

"Shopkeepers will only buy items that are in their "buy" list and only if they have enough money." Page 148 of Ultima Online The Second Age Official Strategy Guide

Edit 1: Vendors also raise the price of items as people buy them because "demand goes up". This was changed June 13, 2001:

"shopkeepers will no longer inflate the prices of their inventory when their stock begins to run low, and prices of items will be consistent on all shopkeepers."

I remember this too but can't say if it ever got changed in era.