i dont think selling diamonds is a good example in this case. they need to be mined an processed to be sold. companies do just automate the buy-process for domains only to resell them for a much higher value.

ticket squatting for example is illegal.

Okay how about bitcoin and all currencies then? people buy them to just up-sell at a higher price. Or stocks, i don't think there is anything wrong with it. Just another way to make money.

And ticket re-selling is a thing, happens in the UK

currencies are in general not a "limited" resource. domains are. also each domain is unique. their use is to register an internet site and not to buy it just to make use of their name which people maybe later will need. i think that is just an abuse of the domain registry