What is "Nope"?
Did you not offer the seller a lower price? Was the original asking price fair and you tried to get a better deal? If already at a good deal, why did you not buy it for asking price? Is there more to your conundrum that what you have stated?
I would understand you asking about this as a moral dilemma if you bought the amp for asking price and were conflicted feeling you lowballed the seller and got away with something. Or if you were a gear flipper and starting having remorse for hosing the seller the way Guitar Center does. Buyers remorse of getting such a good deal and feeling like you stole it or ripped someone off makes sense when asking the morality of that situation and how to handle it.
I'm not trying to be a jerk. You stated it was being listed for well lower than the going rate and you made an offer which implied you offered less than asking price. If you have a conflict based on the morality of taking advantage of this situation or similar then it stands to reason you'd be at odds about paying asking price; feeling bad about that. But you offered lower. This suggests you were not questioning the morality to begin with. Only going by what you stated, the questioned morality is only coming after someone else bought the amp and is in direct opposition to your actions. Would you still be asking this question if the seller accepted your offer? Unless there is more to this, the way it reads is that you are looking for societal affirmation that it's okay to either lowball an already well priced amp or disrupt a sale because someone else got a deal instead of you.
Regardless of what you are actually seeking, I'll comment on the general situation of seeing something priced below its value as I view it. Just like it being on the buyer to do the research so they don't over pay, it's the sellers responsibility to research market value and price accordingly. On top of that there may be other motivations/reasons for pricing. They may just need to move it quickly, they are paying forward the good deal they originally got, they don't care about market value and just want to sell, etc. Whatever the sum of reasonings are for the asking price, the seller has listed it for what they deemed they are willing to sell it for. There's more than enough resources for anyone to figure out market value and it's fair enough to assume the seller has done what they needed to determine the asking price. On the buyers I don't see much coming up in terms of morality of getting a good deal. If questioning the good price; whether you're buying or just came across it, is to ask if they are comfortable with selling for that price. You also have to determine if what the seller is offering is on the up and up. Are they trying to hide something wrong behind what looks like a good deal? If all is legit, you are the one buying, and are wanting the feel goods you can always tell the seller you think you're getting too good of a deal and offer a little more. Unless you know for a fact that a buyer is purposely trying to take advantage of a seller it's not anyone's place to step in and try to disrupt a transaction.
When it comes to the bottom line, there's nothing morally wrong with getting a really good deal. It's the seller who is responsible for due diligence to price accordingly. Just the same as caveat emptor on the buying end. If at the time of transaction both walk away satisfied, then that is that. Both parties will have to accept their own regret if later they find the price wasn't as favorable to them as it could have been and take it a lesson learned. Morality would come into play if the buyer is trying to coerce the seller into thinking what they have is of little value to drive down the price and purposely taking advantage of them, idly standing by knowing a buyer is forcefully pushing a bad transaction on the seller to take advantage of them, or disrupting someone else's purchase because they got the deal instead of you.
