Allowing any trade in endangered species makes it harder to tell whether a given person trading in that endangered species is doing so illegally.

For instance if I have a license to trade 10 animals, then I could try to smuggle in 10 of them. If I get caught, I say, "So sorry, here is my paperwork, I just didn't know who to turn them in to." If I don't get caught, then I go import another 10.

Additionally if there is some legal trade, then smuggled items go up in value because they can now be sold to legitimate stores. The legitimate stores each say that THEIR stuff is legitimate. Looking at the aggregate sales, someone must be lying. Perhaps it was the store. Perhaps it was a poacher who convinced the store as above that, Sure, I have a license for these animals, look right here. 10 animals. You cleaned me out this year! From a law enforcement point of view, even if you figured out who is probably in the wrong, proving it in court is going to be an uphill battle.

An absolute ban works because it makes enforcement easy. You remove all of the shades of grey - if you're selling endangered animals or their parts, you are in the wrong.

I believe that it was concerns exactly like this one which originally lead conservationists to push for absolute bans rather than limited trade. The move back to limited trade is not good. (Doubly so if it is initiated and pushed by people who don't care about the difficulties of enforcement because they don't care about actually seeing this enforced.)

Cheers,
Ben