I buy from eBay, Amazon, B&H photo, several times a month, to save shopping time and travel, with excellent results. I ask questions and read the item description carefully and several times. Once in a long while, if an item is not right, I contact the seller and get a refund. I pay with Paypal as they protect the buyer.
As I use expensive underwater equipment, I often buy and sell stuff via Wetpixel.com (free for its members). Here I also use Paypal for protection and am careful, ask detailed questions and get a feeling for the seller. So far so good, after several years I have had no bad experiences. The occasional returns have gone well. This includes cameras, lenses, water proof camera housings, strobes and other equipment, all have been fairly priced and worked well.
To sell I used eBay, but they now charge me a Seller's monthly fee, plus 10% of the sale price, a bit steep, so I sell my underwater items on Wetpixel. Again, the money goes through Paypal, with no bad experiences thus far after more than 10 years, their fee is worth it.
Moral: Before buying, read everything carefully, ask questions, get the seller to commit himself on important details. After buying, if not satisfied write the seller to state the problem clearly and be polite, keep in mind eBay and Paypal are on your, the buyer's side. If selling, be 100% clear and accurate, price fairly, ship promptly.
For me, buying and selling via the internet is a blessing, as one can shop the world over in a matter of minutes, great variety, excellent prices and service, and at times I meet some real interesting people. Be polite and stay happy, it is good for business and for the coronary arteries: Life is short, enjoy it.