There are items in my inventory that I want to give away, but when I choose 'give personal item' in trick or treat, they don't show up on the list. These items are avi items that are tradeable and are from the current fest, so I don't know why this is happening. I did notice that the avi items on the give-able items list seem to be of epic or more rarity, is that why?

There are quite a few restrictions on what items can be given through Trick or Treating. I do believe that Rare quality Avatar Items are excluded among other certain item types.

Might I ask why? I got an extra starseeker cloak (a rare one) that I'd like to give away to someone instead of sell, among other things that I'd also like to give away.

Because, early on, there were no restrictions and people would just use Trick or Treating as a way to dump unwanted items. Many players were unhappy with this because they preferred a piece of candy instead of random unwanted rare items that they deemed had no value.

Therefore, I enforced some restrictions on what could be given so that the option to gift items still exists, however, hopefully as a more positive experience for players where if they do receive a gift, they will deem it a valuable one and worth it in place of a piece of candy.