Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

How to double-entry bookkeep money incoming from sold items
If you were a business, all your assets would have a dollar value, so when you sold them you'd decrease the amount of assets by that amount and increase in cash, and if there was a profit on the sale it would go in as income, if there was loss it would count as a cost (or a loss)... so if there was a profit it would increase Equity, a loss then it would decrease Equity.  Since it's not really worthwhile doing a estimated cost for everything that you have, I'd just report it as income like you are doing and let the amount of equity increase proportionately. So, implicitly you always had roughly that amount of equity, but some of it was in the form of assets, and now you're liquidating those assets so the amount shows up in GnuCash. When you buy new things you might sell later, you could consider adding them as assets to keep track of this explicitly (but even then you have problems-- the price of things changes with time and you might not want to keep up with those price changes, it's a lot of extra work for a family budget) -- for stuff you already have it's better to treat things as you are doing and just treat the money as income-- it's easier and doesn't really change anything-- you always had that in equity, some of it was just off the books and now you are bringing it into the books.