You seem to be unacquainted with the realities of capitalism.
In a retail business, your customers want to be served by you and are willing to pay for that privilege. They don't particularly care what it takes for you to provide them with the service, or what the consequences to you are if they express displeasure over the value not received for the money spent. You are not a charity that deserves to exist on principle. They have no commitment to you. In a real sense they fire you every time they choose to go to your competitor (be that Walmart, Amazon, or wherever) and feel absolutely no remorse over it.
People move from a contracting relationship to being employed explicitly because they want more security. That's the deal. Firing employees is asymmetric because every time you do it, all of your other employees get concerned that the same may happen to them. Also an employee represents a fairly large sunk cost to you in your search to find one, and training - reasonable employers don't throw that away lightly.
These are at opposite ends of a spectrum. Generally the closer and more complex the relationship between two commercial entities, the more important maintaining that relationship becomes, and the more that you care about maintaining the business relationship. Someone who bought something from you once on eBay doesn't have a relationship to care about, and really doesn't care. An employer/employee relationship is very close and complex, so you care a lot. Any kind of frequent commercial relationship (be it going to a restaurant you go to regularly, dealings with a supplier, etc) will likely be somewhere between those extremes.
There is a great gap between saying "I will not shop with you" and "I wish nobody ever shops with you". I wasn't bothered by GP's desire for accurate feedback, or for consequences for mistakes; I was appalled by their callous attitude.
The attitude was not "I wish nobody ever shops with you", it was, "I don't care if nobody ever shops with you".
This seems quite reasonable. If you're trying to run a store, the details of what is required matter to you, not your customers. If the consequence of negative feedback is that you're replaced by a more competent store, most customers are quite happy with that.
Accurately stating that the seller failed to cover $1.44 of postage is a VALID reason to leave bad feedback.
It is not a valid reason to put someone out of business. If eBay chooses to put someone out of business over a one-line review that reveals a $1.44 oversight, then eBay is the culprit here, not the complainant. The person complaining shouldn't have to care: it's not their fault that others are overreacting.
...and in this case it's not even the seller who forgot the semi-colon. The seller asked if a semi-colon was needed and was told it wasn't, only for the build to break on compile, leaving Bob to edit the source. But the seller offered to edit the source for Bob.
Wow. One small mistake and they should go out of business. Are you treated that way in your job?