I upgraded my MacBook Pro (mid-2009) from 2GB to 4GB when I bought it on CraigsList a few years ago, it was a very good deal, due to a few scratches on the aluminum Unibody case. The seller signed a 48 hour warranty with a photocopy of his drivers licence. (I got burned on CraigsList once before... so I am more cautious now.) No problems, but now I need to upgrade from 4GB to 8GB because 6GB is recommended for running Windows and VMware Fusion... the 4GB was more than adequate for my needs, until now.
I bought an 8GB RAM kit (4GBx2) DDR3 PC3-8500 (1666 non-ECC) 204-pin SODIMM at Fry's Electronics locally, and it would not boot. Took it back, got another brand, and that would run for a few hours between crashes. So, I took that back and bought from Crucial online. The Fry's refund-warranty is only 15 days, and the warranty at crucial.com gives 45 days refund, with lifetime exchange warranty.
Be careful where you buy your RAM, anything less than 30 day return is unsatisfactory, imho. If it works for a few weeks without any problems, it will probably last for many years.
Next up: battery replacement, it only runs for half an hour now, but I rarely use it more than a few minutes when unplugged. The original battery is about four years old and near end of useful life.