Well, your gross revenues are all income, so it's all taxable. But, if you're actually running a business, there are a load of ways to reduce the gross amount. However, that requires a lot of work to file as a business.
I did it an easier way. I had to figure this out a year early thanks to a new Missouri law, thanks lawmakers! I determined that my record collecting is a "hobby" under the tax code. I buy a lot of records, and sometimes I sell records, because I upgraded or I just don't want it anymore. The point is that I don't do it for profit, because I don't, it's just a part of the hobby and I usually lose money. Once you can make this good faith argument, then you are allowed to deduct the legitimate costs of selling - the shipping, the packaging if you buy mailers, fees, and the price you paid for the record originally. Then you simply report the net amount as "other income." There's no specific place for this on a 1040, and my tax program couldn't even help me, I had to do it all by hand. A tax professional would be able to do an even better job with this.
