Hi, I've only sold a couple of things on Ebay, but have bought a bit :B.
If you are buying things, make sure that you read the description, and ask any questions before bidding.
Check the postage. There are a lot of items listed really low prices, but the postage can be astronomical.
If I am buying from a high volume seller, I usually check out thier negative/ neutral feedback through http://toolhaus.org/
This will only bring up the negative or neutral feedback for the ebay Id that you enter, but it has on occasion helped me decide not to buy off a particular seller, especially if there seems to be a common problem experienced from buyers, eg item not as described complaints.
Make sure that you read the sellers instructions fully. I have recieved my first and only negative feedback through this, sending off a moneyorder, and not printing of a copy of the invoice to send with the moneyorder to the seller.
When selling I offer paypal, bank deposit and moneyorder. i havnt had any problems with any of these.
For my selling fees, I have my savings account details entered into the sellers tools, payment options thingy, and the fees are automatically taken out of my account.
When buying things to up my feedback score, it looks like that if I buy two items off someone, I get two feedback comments, but only one feedback rating.
If selling, in the selling form you can choose to block certain types of buyers from bidding on your item. You can block bidders that have had more that one unpaid item dispute in something like the last 60 days, block bidders that have a - feedback (eg 1 positive and 3 negative feedback), or buyers from diffeent countries, if you only want to sell to eg australia.
For payment when buying I look for items with paypal or bank deposit.