We regularly see the question posed in eBay seller groups, on facebook, and on instagram, "Do I need to report my reselling income to the IRS?" Yes, yes you do. In fact, if you sell on eBay for the purposes of making money, the IRS will classify your hobby as a business, no matter how you view it (also, if you buy with the intent to resell you need a business license, but that's a story for another time). Whether you make a single dollar or a whole lot (or even lose money), you run the risk of getting audited and owing the IRS if you fail to file.
Our first year as part time sellers led to an atrocious tax season for us. We had just acquired a business license after already selling for a few months, and had no idea that we needed to be keeping all our receipts (let alone tracking our driving miles, etc). After hours of digging crusty receipts out from underneath our car seats and combing through our bank statements, we were finally pretty confident that we got it right. However, the experience left me with two lasting impressions: If you have to pay taxes on something, you sure as heck better take it seriously if you want it to be worth it, and you also better keep meticulous records. The real reason that we encourage people to keep records is not simply to protect themselves from auditing, but to maximize their deductions. Most small business miss out on thousand of dollars worth of deductions because they are simply unaware and don't track them.