Hi,
most birthday minis don’t have much value but there are exceptions.
The 5 minis from the seventh birthday are valueable: Naga Raincaller, Oni, Shiro'ken Assassin, Zhed Shadowhoof and Vizu.
Some tonics from the sixth birthday are also valueable.
Everything else goes for about 500g–2k, but is hard to sell.
You’re better off dedicating them to your Hall of Monuments and deleting them afterwards if they take too much storage.
You can check the prices in Kamadan Trade Chat.
Most other minis that are not from birthday gifts probably have some value as well. 
To sum it up: Elf, Charr Shaman and Ooze 500g each. EL Tonic M.O.X is up to 1e.
o/
