Reverb (as required by law) issues you a 1099-K and reports it to Uncle Sam only if your annual sales volume through Reverb payments exceeds $20,000 AND you have 200 or more transactions in a calendar year. I think it’d be hard to make a case you’re not doing it as a business at that level of activity.
Massachusetts, on the other hand, requires a 1099 if you sold $600 or more annually. So if the amount is relatively small or only involves a few transactions (like you finally sold that Olympic White ‘62 PB you found in an attic

) then you could easily argue it was all non-taxable personal transactions.
So the total amount and the state you live in will have bearing. A grand or two and a MA address probably won’t catch Uncle’s attention. $20K or better easily could result in a “s’up with that?” query.
The rule seems to be targeting those heavy duty flippers who have flown under the radar for a number of years now. All part of a trend towards closer government scrutiny of online sales activities. As the saying goes: “Great ride! It was fun while it lasted.”