1st of all, check the street price for the EP3 to get a rough idea of how much it is going nowadays. sometime you will be surprise that the price of some seller might even be higher than you get it new from the shops.
After checking on the price, ask yourself if it is worthwhile to get 2nd hand for the saving you will make, I believe no one here can tell you what you would feel comfortable with.
If price is worth while, then meet up with the seller, take the product, and check cosmetically first to see if it matched with the description the seller had claimed. check if all items are included in the sale and if all of them are working fine. Take a couple of test shots and see if it is up to your expectation... make sure you play with all the buttons, scroll and dials and that everything is working fine.
if u are still unsure of what and how to check, bring along a friend that had some knowledge with checking with you.