Buying: A kitchen stove, used, called the seller and he said everything works fine, no problems whatsoever. Arrived there an hour later and some trim was missing, so if stuff boils over, it would run directly into the stove's electrics. D'oh.
I sell stuff more or less constantly via the internet. People have no manners, that is the first thing you learn. Don't hold stuff for anyone. Don't expect people who say: "I buy that, see you tomorrow!" to show up, because for every second time you get that message, they won't show. You ask them: "Well, are you coming" and if you're lucky, there'll be a short reply: "No, changed my mind". Silence on the other end is more commoon. Don't expect people to ever read your ads. "You waited for me to pick up that car for a week, but I read the ad after I said I'd come and found you painted that Volvo's door yourself. I don't want a home-painted car. Why would you want to hear more from me?" Gaaaaah! It's easy to lose faith in mankind by using online classifieds.
On the other hand, you can also make a very quick buck. I am on the lookout for sales on odd items. This fall, I bought a lot of Communist toy/display cars, about 40 in number, got this send in from a sale in Germany. The first 8 sold very quickly and covered all my expenses. Since then, sales have been slow, but I have turned a profit and I have a lot of little weird cars on display in my study - a neat conversation starter. I am also quick to show up at homes were elderly people have either just moved to public care or died. Their heirs usually want all the old furniture gone, and give it away for free. I then sell vintage sofas for 4-500$ each, nice cupboards can bring 2-600$. I once sold a very rare 1930's table for 200$. A full set of 6-8 chairs and a dining table might even give you three zeros, tax free. The downside is that you need to be able to store all this stuff at home, and a proper wagon (e.g. square Volvo) or a van will help.