The difference between a flip that 'feels profitable' and one that actually is comes down to three numbers people forget: marketplace fees, payment processing and shipping. Here is the honest math, with a worked example.
Discogs charges sellers a fee on the item price (and shipping for some memberships) — the commonly cited rate is around 8.5% for the standard seller level, plus payment processing around 3%. Shipping is usually passed to the buyer, but you pay the post office cost plus packing materials.
A $40 record sold with $5 shipping: item fee ~$3.40 + processing ~$1.35 = ~$4.75 in fees before you touch the post office. If you bought the record for $25, your gross margin was $15 — and your real net is closer to $10.25 before the $6 shipping you actually paid.
After the full stack, experienced flippers commonly target at least 50-100% gross markup on known-good sellers just to clear 20-40% net. The sweet spots are: underpriced marketplace listings, well-known pressings bought below market, and bulk collection buys where your average cost per record drops hard.
The trap is volume without accounting: 100 flips at 'a few dollars each' can quietly become a negative year once you total fees and shipping. That is exactly what a per-flip log prevents.
In the US, payment platforms issue 1099-K forms to sellers above IRS thresholds, which have changed over time. Marketplace income is generally reportable regardless of whether you receive a form. Cost basis and shipping/other deductions are where record-keeping tools pay for themselves.
None of this is tax advice — but every serious record seller should keep a per-flip log of purchase price, sale price, fees and shipping. The report tab on this page generates exactly that summary from your entries.