This happens often.
Sure, by posting this thread, you probably can prevent one case. But 5 other overpriced items have probably just been sold to unknowing buyers in the time you've spent confronting him.
I feel that most buyers about to spend money would have done their research, and realised what a second hand version of the subject item should be worth. Not many buyers online would be that easily cheated.
Even so, if they do purchase an overpriced item, and do realise they've been given a bad deal, they've only got themselves to blame for it. Learn their lesson, remember it, and not fall for the same trick again. This is part of life, and part of growing up.
Lastly, no one has the right to fix the value of a second-hand item on the market. Sellers can price their items at whatever price they want. The Sri-Lankan "foreign talent" may have done something which you feel is unethical, but truth is we don't have the right to do anything.
In fact, the word "overpriced" and the phrase "jacking up the price" (or pushing values up) is subjective.
What if i feel that that bass is really worth $250 second hand? and your price of $140 is actually an unusually low price? Does it mean that all other sellers selling the same bass MUST price theirs at $140 just because you did?