My notes from the 2008 Town Hall Meeting in Chicago (and to my knowledge, nothing has changed):
Does the team endorse any third party sites for buying UO gold? Jeremy responded: Buying and selling items for cash in Ultima Online is NOT ILLEGAL. However, UO does not endorse any sellers and completely stays out of it. The head of Mythic is strongly opposed to the sale of in-game items for cash in MMO’s, but UO has long held the stance that it is legal and that will not likely change.
I've hunted and can't find the thread, but I also recall a conversation whereby Jeremy (community person at the time) explained that posting URL's in the game (in books, on parrot names, etc) was not allowed unless the web site was an official UO Fan Site (as defined here:
ORIGIN - Ultima Online - Main).
I've never seen anything that states that players can't talk about cash transactions in-game, since cash transactions aren't illegal. The sketchy part is that most of the sites that offer items/gold for sale for cash also may be using software/scripts that violate the UO terms of service.
I'd conclude that you're safe to talk about cash transactions, but should be careful discussing the particulars of sites that may be scripting and therefore violating the terms of service.