Pics are the best, and only real reassurance you have that its ok. If its all beat up, scratched and what-not... thats the first sign it wasnt taken care of, and if it works, its probably a lot older than they say, and possibly stolen.

A unit that was well taken care of will, in most cases, last you longer and give you better performance.

If its an eBay seller, check their feedback... see if they've sold other car audio equipment and see what the buyer had to say about them. If you want to get really into it, email the buyers and see what their total experience was with the seller.

I know what its like to get shafted with audio equipment from the internet... I bought an amp off someone for $200 (old rockford punch 800) and I got it and it didnt even work!! I ended up getting my $$ back in that case though, luckily!