Do you think it's the internet that introduces the net negatives, or is it just that there are less obstacles and makes bad behaviour surface more easily?

There shouldn't really be a problem with online job postings, but when people on either sides either try to game the system or don't actually want to deal with their job (e.g. HR departsment that can't and won't manually process applications).

Same for you example with the cars. Seems perfectly reasonable to put your used car online and for someone 400km to be interested if it's the right car at the right price. It's only when the hustlers insert themselves problems arise, but that's due to frankly shitty behaviour. The internet just makes it simpler to be a bad or annoying person.