When filing taxes (remember - there's no receipt available) the "profit" would then be the difference between the listing price and the actual sale - correct?
No. The listing price has no play in this.
The government deals only in receipts. If you have no receipt the profit is the entire amount you sold something for. If you have a reciept, you would just take off what you paid for it (but only if filling out a schedule C) and that's your profit..in most garage sales, that's a loss..which of course the government doesn't allow you to take. It's a one way street.
If you do have a receipt, the profit would be the entire amount again...UNLESS you fill out a schedule C, which has places to remove the cost, the fees, etc.
Before eBay, this was seldom a problem (except for the person who made a business out of garages sales). The IRS seldom said a thing, since they figured you are just getting rid of stuff you paid much more for...they could have asked for receipts, but they just didn't...prob didn't want to face the wrath of a nation.
Along comes eBay and online stores, etc, and the IRS sees that people are making a business out of selling old stuff. Will the IRS bother you if they see that you are prob selling your closet stuff (by that, I mean not a lot of multiples of the same thing - what's the chances you have 10 identical toasters in your house..well you prob do CAnn LOL..but most people don't), old clothes, toys, etc)? Prob not. Can I tell my tax clients not to worry about it because the IRS will not bother them? No..because legally, you owe income tax on the profit..and if you don't have a receipt, the profit could be a lot (although almost always on a thread like this, someone will pop in and say their accountant said not to worry about it..of course he wouldn't get in any trouble..just you would). And if you have a recipt, you are suppose to fill out a schedule C or claim the toal as misc income. You really have to do what you feel comfortable with...and take notice I'm not telling you what I would do about income tax on the items from my own attic...since some stuff isn't good to share on a public board.
By the way, I always suggest you go to an accountant the first year, so he can set up your computer depreciation, other depreciations, office in home (and no it's not a red flag if you really have one), and show you what expenses are deductible (gas, postage, ink for printing out auctions, part of your internet cable..all if you use it just for ebay, etc).