You are limited to the number of slot Items and Hand Items you can have in play. Slot Items are Headgear, Footgear, and Armor, and Hand Items should be obvious. Since you only have 3 slots (head, body, feet) and two Hands, you are normally limited to using 4 or 5 (depending on whether you are using two Hand Items or one 2 Hands Item) of those types of Items. You can use any number of Shinies, Clothing, and use/carry any number of belt Items (usually Potions and Scrolls, but anything that isn't a slot Item, requires at least one Hand, and isn't a Shiny or Clothing, goes on your belt). You also have a pack, which can only have a total of two Items you can't use in it. That includes slot Items, Shinies, Clothing, and even belt Items that you can't use for whatever reason. A
Cheat! card always makes something usable by you and will not take up a slot or Hand space. All of this is detailed in the rules on page 16, which has the heading "Items."
According to the rules (p. 15), you can sell Items from your hand to buy a level. However, for a Deal, if it says "Items," it means from those in play unless it also specifies that you can sell stuff from your hand.
In the case of
Ye Olde Donjon Shoppe, I'm currently looking at the production database, and the text is all together. I'll check the physical piece and do a deeper scouring of the rules later, but for now, I'm going to say that the Deal includes both buying and selling, all on one Move. No matter what, you can only do that Deal once per turn.
Edit: After looking over the rules and several room pieces, I see no reason to think that the deal, and others like it, is either one or the other.
Since I've been a moderator on these forums from the beginning, and your question isn't familiar, I don't know to which forums you are referring. Any forum not on this site isn't official, and I'm not even going to look for other discussion forums for this game to see what unofficial responses have been floating about. This site here is the source for official responses.
Edit: Hmmm, I supply a pretty definitive official response in this thread, so it's come up at least once.
In the future, please limit it to one question per thread, or maybe a few related questions (like the ones you asked today) together. So, please ask any new questions not related to my response here in a new thread.