Gary is correct! As of right now (if you check the rules on items, <a href="https://app.roll20.net/forum/post/1213982/rules-on" rel="nofollow">https://app.roll20.net/forum/post/1213982/rules-on</a>... ) "There are very few buying restrictions in this campaign. If you can
afford the list price of the item in the official guide that lists it,
you can purchase it with your GP at any time between quests (or where
you could take an extended rest). Yes, if you want to buy +6
Curseforged armor and you've got 2,125,000 gp, you can. The primary
restriction is that artifacts..." etc. You may buy as many rare items as you like, so long as they are not artifacts or unique items. I have been considering adding in a limit to the number of rares you can have, but I have not yet done so, and given the headache it might cause for players, may never do so. tl;dr: most rare items are perfectly legal, including the helm of seven deaths (which is a cool item!)