Update: This only happens when I buy items from a user shop. Buying from main shops works just fine.
So far I've seen this with Maleria and Cursed quests but I'll add the others I'm having troubles with.
I can't do any quests they give me because when I do, I get this error buying the item for them:

though i can buy whatever other items i want and i have no issues.
Using Google Chrome (up to date) with Windows 8.1
105. it's not an inventory issue as i just discovered that it only happens when i try to buy from a user shop. buying from a main shop works just fine.
There's a limit on inventories. For non GA users, it's 100. I guess you're still able to buy from main shops, but user shops you cannot purchase from if you're over the limit :)
that's weird because it worked fine before (about 2 weeks ago) when i had 141 items. i'll try getting below 100 and seeing if that fixes it.
Did you have a gold account before? Gold accounts can have 250 items
nope! never have had one.
hm interesting. you should have encountered it before then.
huh alright that did fix it. strange that it worked before though! and weird that it only affects buying from user shops.
- They removed the inventory limits while people were opening their Luminaire trees, and for some reason didn't put them back until the past week or so. Everything's working the way it's supposed to now.

Yeah that may explain it if it was longer ago than the op thought, didn't think of that :)
ah alright that makes sense. yeah i joined during all that so i didn't know that the limit was a thing lol