Well when I was first typing this reply I didn't bother to look up current prices so I typed out the part about the 840 off Craigslist which I'd only recommend if you can't afford to buy a new drive. The M500 isn't much of an upgrade from the M4 which is still a great drive, but there are better drives out there in my opinion.
Samsung 840 120GB - $90
Plextor M5S 128GB - $95
Plextor M5P 128GB - $100
Samsung 840 EVO - $105 (Ends 8/21)
Samsung 840 250GB - $166 (Ends 8/21)
Plextor M5S 256GB - $180
Samsung 840 EVO 250GB - $183
Corsair Neutron Series 256GB - $185
If you're still interested in the 840 off Craigslist:
See if the guy from Craigslist will let you test it before buying it. Just open up a program like CrystalDiskInfo and check the SMART attributes. Also run a benchmark like AS SSD or CrystalDiskMark to make sure it performs how it should. If everything looks good I'd buy it. Apart from firmware or controller issues as long as it works it'll usually continue to work till the flash is worn out. Samsung makes great drives, the only drives I've heard about having problems are certain Sandforce based SSDs due to firmware issues which later got addressed and some of the older, crappy OCZ drives like the Petrol.
As far as SMART values here are the main two I'd pay attention to:Warning: Spoiler! (Click to show)
ID # 5 Reallocated Sector Count
Here's a guide on SMART values and what they mean: http://www.samsung.com/global/business/semiconductor/minisite/SSD/global/download/07_Communicating_With_Your_SSD.pdf