You have no idea how many other people are also asking about the same deal.
Irrelevant. In that moment, the OP was a willing buyer. A bird in the hand, yadda yadda...
The "experienced seller" thing is bogus too for the most part, and at any rate not relevant in this situation. The OP is NOT a tire kicker. He wanted the bass and presumably has the money to buy it. He wasn't even asking for a cut on the price, just a minor addition, that the sellers had on hand.
You're right about that. However, if you for some reason were to be corresponding with someone about something you wanted (like for example, asking about a different case), and you actually were interested in buying it, having the listing change before you bought it would make you feel... how?
Well, first of all, it's always on the buyer to confirm the details of the listing before purchase. But also it's incumbent upon the seller to clearly mark any updates to the ad. A smart seller upon receiving a message like the OPs, if he DID in fact have someone "on the line" about the other bass, would reach out to the other person, and say "hey, how interested are you in this bass, I have another guy asking about another bass that this case would also be a match for". Then that potential customer could say "no go ahead, I'm not ready to buy yet" or "no, hang on I'll take the bass as is if you'll do $XXXX" or whatever.
But the rigid, inflexible approach is almost never going to net the seller more money. A situation like above takes him a few minutes of emailing back and forth. And again, you never know. Maybe a buyer can't afford the other bass at $1800 and no one else is even interested, but when you drop to $1750 with the beat up case, it's just within reach and he snags it. So now you sold both basses, and only took maybe a $50 hit to your profit, it took you maybe an extra 30 minutes of work, and now you have all that operating capital to work with to buy more gear. Sell 2 basses for a total of 4450, pay out $500 to each partner, and you have $3450 to put into more gear to sell. OR, wait maybe months or years or maybe never sell 1 or the other bass at all and foolishly let 1 or 2 good buyers slip away out of ego.