For the sake of doing the achievement (and, mind you, so glad I'm done with that one), I'd hoard sell practically anything that didn't have a use to me anymore. The succulents, Basidio spores, Sky Bunny Plushies, tooth pendants, Goldstones, Amber, and occasionally things like dupe companions, surplus of apparel I crafted, ingots like copper or iron that I've got no real use for, Hollow Pineapples, and Nytha Spines would also get sold.
However, I always gave myself a rule. I'd sell them until I only had 100 of each item left. I never fully sold my stacks. Then I'd let things accumulate again, usually into the thousands for the common trinkets, and hoard sell again. Making sure to, at least, keep 100 for myself.
But, really, it's up to you what you want to sell. Some trinkets are used for higher leveled Hunts like fire opal and are rare enough to find that I don't ever touch those in my hoard.
Oh! Almost forgot. And another really good thing to sell are the gems you get from mining. I have 86 ruby gems sitting in my hoard right now. They sell at 4k a pop when you hoard sell them, giving me 344k pebbles! But, now that I have the achievement done I'm less likely to hoard sell. But I hope this helps you in some way to figure out what things you want to sell and what you'd like to keep.
