Basically you file a schedule C with your tax return and deduct auto expenses, up to 25% of your house as "office space that is 100% dedicated to the business" within your house, shipping, ebay fees, credit card /Paypal fees, cost of goods, etc. As a self employeed, you have to pay both sides of FICA (7.65 percent X 2). You need to do an estimated return every quarter and make payments to the IRS for FICA and estimated withholding. Double check local/state business licensing and sales tax permits.
There are a ton of things you can write off being in business for yourself, but you need to keep detailed complete records, expect a greater chance of being audited and know that most of your so called profit will go to fees, taxes, etc. Golden rule is that for every $100 you have in the bank from making a sale, you'll be lucky if $10 of that is actually yours assuming you are selling items for double what you paid and have a good average sale. What I mean by cost/sale size is it doesn't matter if you pay $1 for a widge and sell it for $10 - you still have shipping, packaging, insurance, commission, etc that you have to pay.
I would strongly suggest that you study up on running a small business, especially the tax/licensing/obligations, more so that on MyFico - once you turn your eBay "hobby" into a real eBay "business" lots of things change, especially with how many entities will be reaching out for "their share" of your sweat.
Good luck !
Note, I ran small businesses for almost 40 years, 18 of those as owner-operator and the IRS came after me with a 10 year back audit 3 years after I shut the door and closed the business - results were ugly but even the greatly reduced settlement was painful and I filed honest returns.
