is there a benefit (e.g. achievement) to donating? Not that I'll stop since it's a convenient way to get rid of low-cost items unable to be sold the pawn shop, I'm just curious.

No achievement, does help newbies though if that counts because you don't have to pay for donated items.

there is an achievement for having Humming Power Crystals banked, so you could (unaltruistically?) fragmentize the unpawnable items that you don't want to sell until you reach the top tier of that one I think it's 30K,

If you find something valueless it's probably not very valuable to a new player either Sarah's Quest is pretty generous A case could be made that fragmentizing gets rid of those items instead of leaving them in circulation and that doing this for Achievement is also altruistic. Only some items that appear to be junk will belong to the set of rare items that Subeta is price-supporting and regain value.

thanks. Mods can lock.