I collect skin potions. The designs of the bottles are pretty so even though I'll never change my skin, I collect them. I'm kinda meh on whether I should collect the skin potions done through alchemy. I kinda want to, but it's Alchemy .____.
I still need to get the potions from halloween. But maaaaaaaan that's a lot of stickers and gold.
As you can see in my signature....I'm collecting summer tops. I partly did it out of the heck of it, and partly to get a small chunk of gold when I hit my goal of 500, and partly to get cheap gratification like "haha that's 500 summer tops that is gone from Gaia for good!! Take that DC!" when I do sell them all at once.
I did similar with the blox items (I must have inadvertently copied my sister) with the goal of 100.
I also collect items that remind me of me. Like, I had to have Dreamer's Dust because of my username, and Mwee the Dragon because it is my actual nickname (and part of my email). Stuff like that. Can't say I go out of my way to collect anything else. I don't count my event hoards as collections. More like items that are in limbo as I decide what I want to do with them.
Last year or maybe year before that, I came back from hiatus and stumbled on Revolution.Dusk items. I did a little back and fourth flirting (buying/selling) with Queen and Manslayer and realized just how much I loved them. So I decided to collect them all from Dawn, Dusk and Midnight. Just finished this month actually. So happy the series is over had to sell items I've had since 2004 to finish.