1. Honestly, nothing to stop that happening on Ebay or any other venue
2. Does not happen with Paypal. Once you've provided proof of shipping via a tracking number(which Ebay makes easy for you if you print the shipping label/postage through Paypal), the buyer isn't getting their money back unless they initiate an item not described. Just be sure you always ship with tracking, and be sure you use signature confirmation where appropriate(I think the threshold now is $750 or higher-double check on this).
3. Get the item boxed before the listing and weigh it. Don't be afraid to fudge the numbers up a bit. When listing the item, tell what service you will offer(I suggest priority international-it's expensive for the buyer, but that's not really your problem) and then just input the destination and weight. Ebay usually ends up overcalculating. Or, if you don't want to deal with international shipping, you can specify US only
Honestly, that's about it. Just be sure you photograph and describe the items well, and go overboard with describing anything that could even be remotely perceived as a defect.
Folks will talk about being scammed on Ebay as sellers. I'm at over 12 years and I'd hate to know how much money in that time, and it hasn't happened to me yet. At one time I was a power seller, although now I'm probably down to 15-20 items a year.
I've had unhappy buyers, but USUALLY was able to resolve things and that was the exception rather than the rule(the worst was a buyer who claimed a problem with the item that I don't think was there-I said send it back for refund or repair, they refused and wanted a partial refund, I actually won the case meaning they kept the item and I kept the money, and had the negative feedback they left taken down because I could prove feedback extortion).
If you DO have a buyer complain, just say "return for a refund", give them their money back, and relist.
Also, if selling for someone else, remember to consider ALL the costs. Ebay encourages you to provide "free" domestic shipping, and rewards you with better search placement. Since final value fees are charged on shipping also, it's what I do. I just add the cost of shipping on to either what I want the starting bid to be or on to the buy it now. Paypal takes a cut on the transaction itself(I forget the exact amount, but I think around 3% plus a flat fee), and Ebay charges a 10% final value fee on the sale price(plus shipping). When I sell for someone else as a favor, which is rare but something I do on occasion, I tell them 15% to start with, even though when I give them their cut I give a full break-down and charge actual price. Professional Ebay resellers often charge 25-35%-and the ones who do 25% probably are losing money.
