I'm more a seller than a buyer, slowly reducing my pile of spares and 'bits that will come in handy one day' by responding to WTBs (yes, I should just advertise it and get rid) so my view (I don't really have hard rules, maybe loose guidelines) as a seller:
Good description. Be clear, comprehensive and open about items. I try to be factual and non-emotive in my description of condition, hopefully giving the right info to allow the buyer to judge if the item will meet their need.
Photos. It should be possible nowadays to get good clear images that show enough details of the items. If needed, I'll tell potential buyers that I will send photos the next day as I will take them in daylight (no really suitable loghting in house). I look at some adverts on here (and anywhere really, ebay etc) and wonder if the seller has even looked at the photos and thought 'Are these the best pics I could get?".
Answer all questions, clearly and completely. People generally ask a question because they want to know something.
Good packaging. I keep suitable boxes, Jiffy bags etc from my post, really handy to have. Occasionally, if I don't have something suitable then I'll buy it. Don't just wrap something delicate in a black bin bag and hope it will do.
Let the buyer know when it's posted and ask them to let me know when it's arrived. As a seller, be ready to post: have packaging ready, get it in the post asap or, at least, let the buyer know when you will post it.
Payment: I've only recently got PayPal but have had no problem when I've asked buyers to pay by bank transfer (well, just one. Not a problem as such, but he was all set to buy but then suddenly told me he 'didn't like doing online transactions' or similar. No reassurance from me that quite a few people had done BTs to me on here and I was quite happy for him to put up a post asking for anyone to vouch for me, I would be the one giving him my bank details - not him giving his to me - and that I would be unlikely to offer cash-on-collection from my house if I was going to rip him off, was enought to convince him so he went without the part).
I have told some people that I'm happy for them to pay when they receive the item but almost all have paid straightaway. This is based on the reverse of the guideline 'buy the seller, not the item' (which maybe more buyers could do with adopting). I think the thing is to not blindly trust everyone but, on the other hand, don't automatically be paranoid about everyone and assume they are out to rob you: assess the situation and make a judgement.