The funniest thing I have seen was my fiancé using 1 Frog against 3 Skulls. The opponent kept switching his pets around(same level, same pet), like it was going to matter. Any damage she took, she just healed right up. Then after about 5 switches, the opponent finally forfeited.
On Elune, most level 25 pets kind of just idle on the AH. Personally, you're better off selling it as soon as you obtain whichever pet you want to sell. But, certain pets, like the Son of Animus, and Ji'Kun Hatchling, should sell fairly quickly for a reasonable price at level 25.