I made enough selling Star Wars displays on eBay/Facebook last year that I crossed some threshold that made PayPal send an earnings statement to the IRS, so I had to claim it. The silver lining--I was also able to write-off all Star Wars toy purchases for the year as a business expense. It all worked out, but it was extra work.
I have been hit by a few shady buyers, but they've been the vast minority. I had a guy tell me his package showed up open and empty and he wanted a refund. Luckily for me he was outside the US, so eBay's International Program was on the hook for the refund, and not me. Once they paid him, he never tried to repurchase the "lost" sets, so I feel like it's very likely he scammed the system. I am proud of my work, but I don't feel like it's getting stolen out of the mail...
And I've had eBay add sales tax charges to a few sales, but I haven't been tracking it too much. They don't present it on my side when they do, I have to track it down to see it. I can totally see that impacting whether I make the sale or not going forward.