Some may have been mentoned but my tips for buying are:-
1) Always read feedback comments, feedback rating doesnt always have to be 100% for me to buy but the comments have to be good.
2) ALWAYS pay with credit card thru paypal, dont use money out of you paypal account always transfer this back into your bank account. If you have any probs with anyone you can cancel the credit card payment.
For Selling:-
1) Always ensure anyone who pays by paypal is verifed, should say on the paypal invoice in green or red if unverified. This basically means they have had their address checked against their credit card i believe.
2)Always send recorded delivery/special delivery as if there's any problems you may need to prove you have sent an item. It has to be recorded for paypal or ebay to believe you have sent it, standard parcel receipt will not do, has to have tracking number. This is very very important if the item is of high value.
3)Always keep your postal receipts for months and months as chargebacks can occur months later.
I always try to encourage people to pay using other methods but you cant force them and your items will not go as high if you do not offer paypal as its the safest for the buyer.
Good Luck i have only had two problems ever on ebay, one item was lost in post and one payer used a fraudulant card to pay for my item.