IMHO audio would be a terrible investment, or alternate currency, compared to mutual funds, the stock market, real estate, or investing in classes or an advanced degree to further yourself in your chosen profession. Most of us are in this because we like the music, and occasionally (or even frequently) have a jones to improve our systems to approach whatever "the absolute sound" means to us. You have been fortunate to break even or make a profit on your transactions, and I am sure some here are in it for that reason. However, in my experience, when you throw in your time to pack and ship, shipping and insurance charges, the time to deal with sellers and buyers, the time to search the web for the best deals, and the risk (nothing is risk free) of being defrauded or losing on shipping damage, you would likely do better in any number of "businesses." I feel I have been fortunate to do ok at this by buying only components I wanted to upgrade my system, sticking to items in good condition, being patient when both buying or selling, getting to know the sellers I am buying from, and walking if things do not sound right. Even so, I have had 3 deals where I felt the item was not in the condition advertised, and 2 deals with shipping damage. The shipping damage deals turned out alright (after significant time involvement) and one of the disputed condition items turned out alright after I e-mailed pictures to the seller (who had been selling for a relative and had not seen the item). On the other 2 deals, I took a loss on resale since I described the item honestly and accurately. Believe me, you will do better to simply have fun with this, enjoy your successes, take your disappointments with a grain of salt, and find a better way to make money.
as if not outright then by receiving an item which is not in the condition advertised
