Re:
There are several sides to this IMO and in some cases it plays out poorly but in others buyers are victims of their own greed.
An ad pops up offering something for 100 that's worth at least 250. Potential buyer gets the usual, exciting, rush of blood to the head and sends a message agreeing to pay the 100 (knowing full well it's worth more). At the same time other, slower, buyers start circling and sending messages. Seller confirms sold so they offer more. Let's say 150. Seller does what a lot of people would do a takes better deal.
Who's at fault here? This isn't a charitable regulated industry remember...
If first buyer is fairer/kinder and offers closer to market value they'd be more likely get the deal but no, they want the bargain so should accept the risk.
The second buyer owes the first nothing so why not make an offer?
The seller should maybe stick to their first deal but if the first buyer is choosing not to alert them to their under pricing then I understand why they'd take the higher price.
Yes they could do better research but if someone's clueless older parent is selling then would they know where to start?
If this was someone selling a car for 1000 that was worth 2500 or 10000 that was worth 25000 I'd expect it to go through offers even if the first was was accepted.
On one hand people want others to act with honour and stick to the first deal but they are less keen to act with honour themselves and tell the seller what it's really worth.
As for flipping it is what it is, again it's not a charity!
All I can say is I've lost out when trying to bag a bargain but never when paying a fair price.