Yep, as B said most of us want to keep selling, trading and buying on here so have reputations to think of to be honest.
Be straight and offer a money back if not happy kind of deal and take a good couple of decent pics so you know and the other party know what you're getting. It's all easy from there. A photobucket account and a half decent camera helps - even modern phones have decent camera resolution on them so no excuse for those shite fuzzy ebay covering something up type shots which still appear on here every now and then with the usual "sorry about the crap phone pics" excuse - frankly you can buy a 5Mp camera for less than the cost of the tyre you're selling is my instant suspicion when I see these kinda adds!
Rant over... ;-)
Also handy to trade with folk who offer to meet or who exist in the real world and go to meets like Blackpool, Creation, Radchester etc as they have more to loose if they try and fook you over as you can have it out with them face to face!