I work in a PC shop and I can tell you from experience that not all RAM of the same specs will work in the same board. Often its a matter of going back and trying a different brand or batch of memory. We once had two friends buying identical machines except for the memory, neither board would post. Swapped memory between the two boards, both came up perfect and are still running today.
The first shop sounds like they didn't have that memory in stock and really wanted to sell her a new machine. If it was RDRAM I would probably told her that a newer used machine would be the cheaper option.
But I haven't had great experience with PC shops either. My wife was looking for something to get me that I could "actually use" this past Christmas and I suggested I could use some RAM for my slightly older PC (needed two 1GB memory modules). I did my research and looked up the exact type of memory modules that my motherboard needed from the manufacturer's website and manual. I wrote down the specs (an exact quote from the manual) on a piece paper for my wife. Since these were going to be used parts, I directed her to the local "mom and pop" computer store which I normally liked to support instead of the "big box" stores.
She went in one day after work and explained that she would like to buy some older RAM modules and that she didn't know much about much about "computer jargon" but that (her husband) had written down what he needed. The fellow took the paper and said something like "well it looks like he doesn't know much about computer jargon either" as if what I had written down was jibberish. He then tried to tell her that it wouldn't be compatible but that she could buy such and such a new part (this was the owner of the store who had run it for over 10 years).
I was pretty frustrated with this arrogant response. I took the specs, went personally to ANOTHER computer store (I won't be going back to that first store again) and within 20 minutes had returned with exactly the RAM modules I needed and had my PC's memory upgraded. As a bonus, it was cheaper at the second store.
