RAM is probably one of the few items I would feel confident buying used provided it is from a reputable seller and a reputable brand name.
I would, however, run Memtest 86+ on it as soon as I get it, I would stay away from China because you might be getting knock-offs, and I would stay away from High-Density junk RAM modules.
I have tons of used RAM and it is most all good as far as I know. Generally, it simply outlasts the usefullness of the computer it is in.
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I would also buy a used CPU provided it is clean, comes with straight pins, and is guaranteed to work. I mean, I have supported thousands of computers and seen only about one (1) bad CPU in my life. Usually if there is a problem it is because someone over heated it or bent the pins.
Motherboards. I would avoid like the plague. New ones are unreliable enough. That said, if a manufacturer such as Dell or HP sends me a Refurbished board or sells me one because that is what they have, I buy it without hesitation.
I personally wouldn't buy hard drives that are used because new ones are unreliable enough and you can't really trust the motives of every seller...
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What is your budget for 2 GB DDR 1 RAM? I am selling some on eBay. Right now it is at $30 + $1.30 shipping... Don't worry it's good quality and in good shape.