Prices will go down to default levels when the vendors reset, which typically only happens in towns with certain types of publishes. When Publish 56 goes live, odds are, the prices will go back in towns where events will occur.
Also, Yellow NPCs can be killed by certain event NPCs (like the berserker Daemons and the Ophid Berserkers before them). If you can survive in a town during an event well enough to buy resources, you can lead one over to an overpriced vendor, invis, have it kill the vendor (the replacement will spawn with the default prices), then kill the Berserker.
I did this several times to get cheap leather, while training some of my tailors, during the invasion of Nujelm eearlier this year.
Note that some items have a static "buy-back" cost that is woefully low. This is a result of some people figuring out a way to generate gold through an exploit with the old system (Where the buyback cost was always 1 less than the sell cost). However, that fix royally screwed Nix in Doom (as there were people who would go "sell down" the bandages, arrows & bolts for profit, in the manner that the system was actually intended to work prior to the exploit - that no longer could do so after the change).
