Did you sell your personal stereo gear, and can prove you paid for them? Then there is no tax due if you sold for less than what you paid for. If you sold your personal items for more than what you paid, then it’s a capital gain and taxes are owed. But the biggest trap here is that the items you sold you didn’t plan on selling. If you planned on it, then the IRS can label you a sole proprietorship and that’s a whole different ballgame. Then the locals come sniffing around for sales taxes. Talk to a CPA and be honest.