This is a little necro bump, but this is something that should probably be clear to people engaging in buying and selling items frequently. To get it out of the way right off or if you don't have time to read the rest of this post just understand this:
No matter what is sold or how often it is sold, all US taxpayers who make a profit on a sale are obligated to report that money as taxable income according to the IRS.
Now, the most important part is the word "profit" as that is what determines if you have to report the sale or not, and what your intent with said sale is determines how that profit is reported. There are three possible ways for a sale to be reported, and I have outlined them below as well as their potential Federal tax ramifications.
Capital gains - While it's possible that your sale may be treated this way, it's rather unlikely. This would be used for an item that you sell at a profit, but the caveat is that it generally needs to be an investment or property of some kind to qualify. Generally, this kind of tax treatment is reserved for things like stocks, land, houses being flipped, or second homes being sold but I could certainly make a case for certain keyboard related things being treated this way. People joke about how they should have purchased Clacks years ago when they were sold for $10-20 and held onto them to flip, but in a scenario where someone did exactly that with intent to treat those purchases as investments I personally would report those profits as capital gains.
Tax ramifications here are going to be the most advantageous for most people as long as the items are held onto for at least a year before they are sold. If they are held for a year this results in 0% tax as long as you remain in the 10% or 15% tax brackets, which if you file single and make less than $47,500 or file married and make less than $95,000 you probably fall into those tax brackets. If you don't hold onto the item for a year, the gain or profit on the item would be taxed at whatever your personal income tax rate is. You are also allowed to offset your gain with expenses directly related to your item, e.g. if you purchased a Clack years ago for $20 but paid $5 to have it shipped to you and sold it recently for $200, your net gain would be $200 - $20 + $5 = $175. With personal-use property (pretty self-explanatory but that just means things you use personally), you CANNOT take a loss but if you have any gains you are supposed to report them. This means if you lose money selling a keyboard you can't take the loss but if you make money selling a keyboard you are supposed to report the gain as such.
Hobby income - The most likely scenario for people buying and selling keyboard related things will probably be hobby income. Hobby income can be hard to differentiate from self-employment income, however, a good rule of thumb to use is the hobbies have no real profit intent while businesses do. There are many, many other things that the IRS takes into account when deciding if you are a business including but certainly not limited to your expertise, your history with similar activities, the manner in which you carry on the activity, and the time and effort you put into the activity.
Tax ramifications are going to be middle-of-the-road here, as you will just be taxed at your personal income tax rate on your profit. With how hobby income is calculated, you are supposed to report that income on line 21 (Other income) of your Federal tax return but you can offset that income directly on that line with whatever your purchases costs are. So for an example, if you sold 10 keyboards for a total of $3000 but paid $2500 for the keyboards, you would report $500 on line 21 as Other income. If you have additional costs you want to deduct such as shipping or mileage, you have to file those on a Schedule A as part of your Misc. Itemized Deductions which is also subject to a 2% of your income threshold up to the point of breaking even against your profit. With hobby income, you CANNOT take a loss to offset your other income but you are allowed to zero out to show no profit or loss.
Self-employed income - Worst case scenario but also probably the least likely scenario, although the line between SE income and hobby income often blurs a little bit. Self-employed income is income you are reporting on a Schedule C because you are in the business of doing something with a clear profit motive. This would be how I would classify people who enter every raffle that they can with the intent of flipping whatever they win for much more than what they paid for it, or even people who do the same thing with keysets.
Tax ramifications here are going to be the worst, with you paying whatever your personal income tax rate is plus an additional 15.3% self-employment tax on your business profit. The additional 15.3% comes from Social Security and Medicare taxes which are usually taken out of your check if you are a W2 employee, plus any applicable State taxes assuming you reside in a State with income taxes. The nice thing about self-employed income though is that you write expenses off directly against any profit you have on the Schedule C, and you CAN take a loss if your expenses exceed your income which will then lower your overall personal tax liability. Beware of taking losses too much though, as if you fail to show a profit for 2 out of the last 5 years the IRS says you should defer to hobby treatment of your income unless you can provide them with something showing you indeed have a profit motive.
Now, I am not the tax police nor do I know everything there is to know about US tax law, I am just trying to lay out what the IRS wants people to do with the transactions they are making and how they should be reported.
