To me, every situation is different. In this case, since the seller is a minor, I would tell them what they had, and offer what I could afford to pay, etc. Moral issues aside, buying what could be a family heirloom from a minor might have legal ramifications. If the seller was an adult, I would pay their asking price, no questions asked, no regrets, everyone is happy. An adult has every resource available to research price, value, etc.
On the other hand, as the owner of a vintage drum business / website, I have opportunities on a daily basis to purchase vintage drums for very, very cheap prices. People constantly write through my website with a drum, they don't know anything about it, and they want to sell it. I could tell them any price, and they would take it, because they trust my opinion as a vintage drum dealer. Wood dynas, black beauties, old K's, you name it. There's tons of the stuff out there, but a lot of people don't want to deal with ebay or craigslist. In these cases I ALWAYS tell them what the item is worth, and if they are selling, I tell them what I can afford to pay in order to make a profit.
But with craigslist, ebay, garage sales, etc, these are public market sources, and sellers have had every opportunity to research their item's value, and like Ploughman, I never lose a minute of sleep over great deals. Only a couple of times have I turned down deals for moral reasons...one time a guy was selling a 60's black oyster Ringo kit for $250, and I told him I was on my way with cash. He told me that he had cancer, had lost his job, and was now selling everything he owned to pay the medical bills! That was too much for me...I told him to put them on ebay, and that he would make at least $3K. He was thankful for the info, and did put them on ebay, so I was glad I could help. But situations like that are very rare!