I was thinking this^. It's called "capital gains tax", and it's worth reading up on. I know about it only in terms of buying and selling houses.
With houses anyways (and various other valuables), it works like this- say you buy a house for $100K, and you sell it for $200K, you don't have to pay taxes on the full $200K, you only have to pay taxes on the profit ($100K). And Spey is correct about not having to pay if you break even on the sales or if you lose money. However, there are other important details, like for example, you have to own the item for a certain amount of time before you sell it. But this is why people hire accountants and tax experts, to figure out all the details of the tax laws.
In regards to avoiding PayPal and similar invasive options, there's always USPS money orders. I see a lot of people selling knives online who only accept money orders. If I were going to sell knives online that's what I would use. It's not as convenient, but you don't have to let anyone (PayPal, etc) get up in your business.