A good deal is what saleroom may be offering (4)
I believe the answer is:
lots
I believe this is a double definition.
'a good deal' is the first definition.
(I've seen this in another clue)
'what saleroom may be offering' is the second definition.
(as in lots in auction)
'is' is the link.
(Other definitions for lots that I've seen before include "Many found in slot" , "may be drawn" , "Loads" , "Large number or amount" , "Great quantities" .)