You definitely have to be careful. I don't sell big ticket items and/or anything where I am not prepared to just lose it all together anymore. It's just way too easy to get burned. I've been fairly lucky, but I am also pretty cautious.
I had a person recently try to rip me off on a Nintendo Gamecube. Buyer had a 0 rating which made my spidey sense buzz a little more than usual right out of the gate. I tested it at a local game shop before shipping it out as I didn't have a game of my own to try it with and the system worked perfectly. When I was packaging it up, I had an extra box and in an odd coincidence I ended up double boxing it and packing it up insanely securely, way more than I normally would have. Plus the system was only going from St. Paul MN to Chicago. 2 days later with no communication I get a return request filed. Buyer said the disc doesn't even spin and the system doesn't work. This didn't pass the smell test, there was NO WAY this could have possibly been damaged in shipping the way I packed it. It was obvious to me the person was trying the old switcheroo with their broken system. I responded to the claim and asked for more details and tried to help...no response. So I wrote again saying he could return the system, but that I had already forwarded on to eBay the serial number from the unit, so I was just letting him know as a word of warning that if he didn't return the original system, they wouldn't give him his money back and he would lose the item. Sure enough, he never sent the item back, the time to return passed and I called eBay and they closed it out and released my money.
As a buyer the worst situation was when I bought a stone waterfall garden feature. Seller said it was brand new, but didn't have a good picture, so I was very skeptical with the price they were selling it for and that they didn't seem to know much of anything about it. Never could get better pictures, but we settled on $120 so I figured it was worth a shot. This thing was huge and heavy, so I knew it was going to cost at least half that to ship. Well, it shows up and may have been new, but the original packaging material had been removed. Needless to say it showed up crushed and damaged in a few spots without any packaging in the original box. I wasn't very happy, because dealing with the seller was a hassle and in a way I could see this coming to a certain degree...I just hoped I was wrong. I sent him an e-mail about the damage and didn't get a response, so I filed a return request and added pictures. Got a response right away on that. They ended up just refunding me all the money and letting me keep the item as there was no way they wanted to pay again to ship that thing back to them.
The biggest scam nowadays I have issues with is people buying things broken/as-is/untested, trying to fix them and then if they can't fix it, they open a return request...even if you say no returns. They get items cheap this way and manipulate eBay to have a guaranteed return policy in the case they can't fix it. It's a no lose situation for them. Buy cheap, fix and resell for way more or just return the item if they can't fix it and get all their money back.