Yes - did you think that they exist for our benefit?
It can be argued of course that if the dealers didn't advertise on them then they would advertise somewhere else, but the fact remains that advertising on these sites is extremely expensive, and inevitably pushes prices up a lot.
it also automates and therefore de-skills the selling process, which means that anyone can sell anything without any specialised skills and knowledge.
Buying insurance? Just give the site all your personal data, and the prices will come back instantly, no need to speak to anyone who actually understands the insurance business and who knows who to ring up to get the insurance cover that's right for you.
Buying a camera? Just go for the cheapest online price. You may be able to find a traditional camera shop, staffed by experienced people who are photographers themselves, but they are an anachronism, struggling to compete and doomed to fail, when rents and rates increase, minimum pay increases again and the local shoplifters have a good day . . .
These were once real, viable businesses, but the internet in general and grey sellers in particular have turned them into lifestyle businesses, when the existing, knowledgeable staff retire they won't be able to replace them, the only reason that most lifestyle businesses still exist is that they owners don't know what else they can do, and the business has become little more than a kind of a paid hobby. Other typical examples are model shops, remote country pubs that don't have any customers who can walk there. We have a small farm, the nearest pub is 4 1/2 miles away and there is no public transport, I'm guessing that the pub would close pretty quickly if there were enough police to stop their customers when they were driving home . . .