I'm not a tax lawyer (etc.), but the IRS has different rules for hobby income:
Tax Tip 2017-80, November 21, 2017
www.irs.gov
If your hobby is knife collecting, and you only ever sell knives at a loss, then you'd claim the income from selling them and then deduct all of it as hobby expenses. Hobby expenses can't exceed hobby income, and you can't use those deductions to reduce your other income.
So provided you already itemize your deductions, this sounds like about 2 minutes of work and shouldn't incur any additional taxes. (
Again, I want to stress that I'm not anyone's tax lawyer. Anyone planning to do this should do some homework. 
)
If your knife-collecting hobby
is a business, you have to follow the rules for operating a business. This has some benefits, such as being able to deduct business expenses (e.g. you need your computer to sell knives on the internet, but while buying a computer isn't a knife-collecting hobby expense it
is a business expense), but also some downsides, like taking inventory, recordkeeping, etc.