If you expense the items, then your "cost basis" is zero (cost basis is, roughly speaking, the amount of already-taxed dollars invested in something). When you sell something with a zero cost basis, the entire proceeds will be taxable business income.
For various reasons that I don't really want to take the time to delve into, expensing small items that fall under the safe harbor will almost always be better than listing them as assets and depreciating them, even though you may have to report some taxable income if you sell the (used) items later.
