The new IRS requires all credit card or similar agencies (PayPal, Google Checkout and Amazon, etc) to report any person or entity that receives 200 exchanges and $20,000 in value for tax year 2011 (NOW).
I have seen the 200/$20,000 figures reported as either/or but now that both amounts must be met, so read this to get more answers or be sure to talk to a tax consultant. http://edocket.access.gpo.gov/2010/pdf/2010-20200.pdf
If you think you may be even close to those figures, it would be wise to keep track of expenses incurred in your selling endeavors. It is wise to do that anyway, but eBay may seem more like a hobby right now, so you are not thinking as a business.
I know, I know, it is not a law that I am fond of either, but it seems to be one of the inevitable things in life and I am not going to live in fear of 'the Man' making my life miserable.
One of the 'gotcha's' in this law seems to be that they will have to report ALL fees paid to you as a vendor. That will include shipping which at times can be more that 50% of the amount, so pay attention!
They say that cash transfers, loans cash advances will not be part of this, but mistakes happen, or directives are misinterpreted, so.......