Yeah something that has been missed in this thread is that the IRS has started to require Paypal to register all eBay sellers to file 1099-K forms starting last year which is the equivalent of a business tax form. If you make $20,000 or are projected to make that much during the year, Paypal will release your sales info to the IRS without your consent. If you make a lot less than that you won't need to worry about it. For the average taxpayer, you will essentially have to pay 30% of your profits to the IRS or at the very least have it deducted from your usual tax return. So if you make a $100 profit, you will basically owe $30 to the IRS next year. Just keep that in mind in calculating the worth and hassle.
Filing the the tax forms itself is relatively painless using standard software like TurboTax. The issue is if the 30% loss is worth it or not. As a former auditor myself, I can tell you once you're caught up in reporting business income, you shouldn't be surprised if your city sends you a tax letter requiring you to file as an official business which can be about another $150 per year in registration fees. Also going forward you'll be expected to report all of your eBay profits. It looks mighty weird to reports thousands of dollars in profit in 2013 and then nothing the following year. There's always the risk you get audited at that point.