Do certain merchants in Gothic 3 actually pay increased money for certain items?

On the internet, people often claim that. For example, hunters buy hunting trophies for x2 price compared to other traders, and thieves pay more for jewelry. However, I was unable to confirm this: any items that I've tested turn out to have the same price when trading with Kliff, Sebastian, Marlo, Seruk, and Urkrass.

I am, however, using Content Mod, Quest Pack, and Alternative Balancing of the Community Patch. Did they change item pricing? Or are special prices for certain items originally nothing but an illusion?

Certain items fetch an icreased price at certain NPCs originally.

See here

Translation:

Hunters and fences pay 30% base price extra (so 60% instead of 30%) for certain items. Hunters pay extra for hides and pelts. Fences pay extra for silver and gold goblets and coffers as well as rings and necklaces.

The sources are relatively unclear on jewelry. Some sources mention only goblets and coffers, others also mention jewelry. It may be, that the extra money for jewelry was removed in the CP or restricted to simple silver and gold rings. At least at some point the CP restricted the hunters extra payment to only pelts and hides, originally thery also paid extra for claws and horns.

Going by your list only Marlo is a fence. Kliff, Sebastian and Urkrass are standard merchants. Seruk is an additional merchant added by your mods, he doesn't sell or buy anything in the original game. This is why he is not marked as a fence.

So, I think, it's simply a case of you not talking to any hunter and not having the correct items for fences.