Bicycle Mechanics - Buying a used Cassette and Chain

I've been in the market to buy a new cassette to change the gearing from my current one. I came across this listing on CL, Used sram red cassette and chain. It's 11-25 which is what I wanted and looks like a good price. Question is how do I know how much life these parts have? Is buying these parts used a risk not worth taking?

http://miami.craigslist.org/mdc/bik/3146599495.html

Thanks

Not worth it....pass it up.

I would never consider buying a used cassette and chain unless I was certain they were very low miles and the chain was long enough to work on my bike. General advice; buy new.

I would sooner buy a used toothbrush.

Nope, don't do it. Chances are both are well worn. Whenever I put on a new cassette I always put on a new chain at the same time. I think you get a quieter, less problematic drive train when parts break in and wear together. My old chains and cassettes either get donated to the bike co-op or put on flip bikes if they are still in serviceable condition.

1. You can't tell if the chain is worn out.
2. You can't tell if the cassette is worn out.
3. The chain has already been cut, and may be too short to fit your bike's large-cog/large-chainring/chainstay-length combination.