It's pretty much a no-brainer when it comes to deciding on this one. It obviously happens often enough to get many users incredibly annoyed by it (frankly, once is enough for that).
Notes on the points mentioned so far:
1) "Yes, we need confirmations when accepting a pet trade too.
Not to mention a whole lot of other things, like sending items or tu to folks, or using quickstock to send many items at once."
We need confirmations for anything that involves the movement of pets or items to another user. Some things, such as item trades and collections already have things like this, so there is no reason why pet trades, item sending, etc should not. Especially when transporting valuable items or large amounts of tu to another player, the lack of a confirmation could easily cause you to transfer those items to the wrong user, or even to nobody at all.
I've seen people type ina username that they thought to be correct, only to find that they were missing a letter. The user they sent the items to did not exist, but they had still lost the items. Because of this, we need at least a confirmation there. It should not be possible to send items into nowhereland.
2) "I want a way to block all incoming pet trades."
Most definitely. You can easily unblock them whenever you have planned pet trades, and this way there is absolutely no way that someone can randomly offer on one of your prized pets.
3) "Or I want this arranged so the person has to request the ability to trade with your pet."
This would also work. This way you will know if it was a person you planned to trade with or not and there is no way to run into messy situations. Another option might be that a person only has to request to trade with you once, and then they can trade with you as many times as they would like. However, the person who allowed them to trade in the first place would also have the option to reblock that person from trading until next time if they don't know them very well.
4) "Would be coolest if you could block each pet for itself instead of all at once. (for me at least, sometimes i do want to trade with some. X3)"
I have seen some people lately that are asking people to set up trades with any pet on their profile that they want "except for so-and-so". Like Mia said some people, for whatever reason, do have pets on their profile that they are willing to trade. It might be nice to be able to select which pets you do not want to be traded. There would be a notice in the pet's viewing box, as well as under their image in the All Pets module, that said NOT FOR SALE OR TRADE or something along those lines. If someone tried to set up a trade with that pet, they would get the same notice. (Seems like I seen this suggestion somewhere, go ahead and take credit for it if it was you.)
Overall, in some shape or form, these ideas need to be put into play. I hate seeing things go wrong with valuable pets and items without confirmations and blocking options. These options would take away a large source of annoyance and be completely helpful.