Oh, no. Never take tax advice off a public forum of non-tax professionals!
An item's value does NOT generate taxable income by merit of being retained by the purchaser -- IF the purchase was a REQUIRED portion of your job.
They are "reimbursed business expenses", which are NOT taxable.
However, if your purchase is of some value (I'm not talking $1 candy bar at the gas stations, folks; more like a Coach wallet, for example), and you list it on e-bay and sell it, the funds from that resale IS taxable income. With a cost basis of $0, as you were reimbursed for the item.
Technically, if you purchase, then return for full credit an item for which you receive reimbursement, yes, indeed, the refund is taxable income.
Theoretically, anyway. In reality, if you're talking about a few hundred dollars over the course of the tax year, the chances of being caught are (let's see if I can spell this) infinitesimal. If you do this regularly, I would suggest merely reporting the gross on the 1099 and not deducting the "reimbursed business expense" on your Schedule C. That way, you're reporting and paying tax on all the funds received, without overburdening yourself with the required bookkeeping elements.
I recently contemplated copy & paste an article on self-employed "gig" workers for taxes. As I read it, I found it so full of errors that it could mislead readers into egregious tax errors. And this article was from Forbes, who should have known better! So, I say again, do NOT take tax advice from non-professionals!