In July, Mr. B adopt 15 pets, which has three variants.
8 of them grow into Variant 1 (common),
5 of them grow into Variant 2 (uncommon), and
2 of them grown into Variant 3 (rare).
Time passes, and it's now November. All of the Julys variants are now rare. Mr B. have traded 6 of V1 and 3 of V2 in the earlier months, Mr. B is left with all three variants, with a duplicate each.
Everyday, Mr. B receive trades from new members, offering to trade (or sometimes, hoping that he'll give them) his Julys for newer pets. Mr. B had made it clear that he won't trade them unless it's a fair trade, but the offers still persist. It's getting really annoying.
That why I suggest that we should make pets non-tradeable after a certain period of time (say, 100 days/3 months after you adopted the pet). After that period, the pets won't be tradeable, they'll stay in the account, though they can still be deleted.
Also, when the pets are fast approaching that period, and the owners still have duplicates (and he/she won't want them in his/her account any more), he/she could trade them off to newer members (Julys for Novembers). Thus the newer members could have old pets, and the original owner will be assured that the new members won't trade them away.
It is possible?