I sell on Ebay &
Amazon as a home-based business. According to my accountant, I can only deduct the cost of merchandise that is actually sold in a particular tax year (I can't deduct the cost of merchandise that I paid for but haven't sold). I also deduct Ebay, Amazon, and Paypal fees, postage, shipping supplies, and mileage. I do collect Florida sales tax and remit it quarterly (usually only about 10 bucks a quarter, since most of my customers are not in Florida).
I keep track of all my sales on a form I print out from my computer and keep the forms in a notebook. I record where the item was sold (Ebay or Amazon), the date sold, the price I sold it for, how much I paid for the item, the date I was paid and how (Paypal, check ,etc.) & the name and address of the person who bought it. I prefer being able to flip through a notebook rather than using a spreadsheet.
I use a spiral notebook to keep track of the merchandise I buy & how much I pay for it (I've read that the IRS is fine with tracking purchases in a notebook if you don't have receipts--for instance, if you buy your merchandise at yard sales).
At the end of the year, I add up all the money I took in and subtract all my expenses.
My DH repairs, upgrades, and networks computers as a home-based business, and he deducts mileage, cell phone, Yellow Pages advertising, etc. He also pays quarterly estimated taxes to the IRS. We don't deduct for a home office because I've heard that's a red flag for getting audited. Plus our offices are a very small part of the square footage of our home.
I also have a "day job," so not all of our income is self-employment income. We always did our own taxes until we started having our home businesses--while we do all the calculations of income & expenditures ourselves, we give this to the accountant and let him figure out our taxes and fill out all the forms. He only charges $150.00 for this and it's worth every penny to us.
Bottom line about selling on line--there is an electronic record of EVERY transaction you make. A guy who used to post regularly to an Ebay newsgroup was audited by the IRS (for reasons related to real estate sales, not Ebay) and he said the IRS auditor walked in to the meeting with a printout of ALL his Ebay sales. Luckily, he had been reporting them and paying taxes on them. My feeling is that if a person is ever audited, they could be in trouble for not reporting income they earned online.