Clarifying this by using proper PhotoPost variable names:
The user's groupname should be displayed next to their user name in the showproduct "For Sale by" section of each ad. Along with their Registered date, Location and other facts, this would assist a potential buyer to gain confidence in the seller and allow the readers to know who's who.
Granted that this will take extra queries from PP_users and PP_usergroups to use $usergroupid in showproduct.php however, it is still valuable information to the buyer.
Keep in mind that I am running PP Classifieds in stand-alone mode with its own user database, NOT associated with vBulletin. The more I test PP Classifieds, I find that I am quite comfortable keeping its users separated from my vBulletin boards, which are on another domain name.