Thanks Mr. Jad. Somehow managed to get all the zombie trophies... :/
I'd be happy to help you out though!
"The Collector" wasn't too bad if you just focus on buying off the walls instead of the random box.
"Hands off the Merchandise" was actually very easy going solo if you follow this walkthrough:
YouTube - Call of Duty Black Ops - Hands Off the Merchandise.
The other two are pretty much up to chance whether or not you get the right weapons. Once you have them, the execution is fairly straightforward.