Wenchy and Barry make good points. Their posts sparked an idea in my tiny brain that might work, but I haven't had time to think through the possible repercussions.
Instead of vendor-based pet deeds, how about adding pet selling (or at least advertising) through the stables? To keep it simple, let's say this would only be used for advertising. Here's how it might work:
1) Add an option to the "claim list" gump, and/or add a new stablemaster-recognized command, that would allow a tamer to advertise their pet for sale. Once in this state, the pet would be added to a master list - categorized by pet type - that was accessible at any NPC stable. Sam the Tamer would be able to add a short comment about his for-sale pet (e.g., "0.8 GD, great for mongbat farming! Contact me at ICQ XXX").
2) For prospective buyers, add an option to the stablemaster gump (the one that includes buy, sell, train animal taming, etc.) and a new stablemaster-recognized command called "view advertised pets" or something similar.
3) Prospective pet buyer Gladys, on the lookout for a 5-slot pet to hunt mongbats with, would go to any NPC stable, activate the "view advertised pets" function, and be presented with a categorized list of all pets for sale on the shard. Gladys could drill down into the greater dragon list, select Sam's GD, and view the GD's lore gump as well as Sam's comment about the GD. I have to tell you, Gladys is VERY excited about Sam's GD!
4) If Gladys wanted to purchase Sam's GD, there could be an "I'm interested" toggle on the gump that would allow Gladys to leave her contact info so Sam could get in touch with her. Not sure what would work best here, but in any case there would be some mechanism that would allow Sam and Gladys to connect and discuss the potential sale.
5) Sam and Gladys make contact, meet at WBB (they both fondly remember the old days), and complete the transaction. Sam is happy to free up a stable slot and Gladys can hardly wait to start farming mongbats with her new pet!
A system like this might address the two big issues here: helping tamers advertise pets they want to sell while also ensuring that sales will still happen the old fashioned way: face to face. Maybe it's too complicated - if so, I'm guessing veteran tamers out there will come up with something more workable.
Thanks for listening!