Is "Hop2" legitimate?

it has reviews out there
https://www.trustpilot.com/review/hop2.com

It looks like a fairly recent startup along the lines of Orbitz or Expedia.
It's hard to say if they're legitimate. I'm sure they would assure you they are.
But if an item or service is offered at an "unbelievably cheap" price from an unfamiliar vendor, one would generally be well-advised to be careful.

Just go to that airline's web site and book the flight. A legitimate third party is not going to be able to sell the ticket at a lower cost than the airline. If it does, the ticket is worth about the price of the paper it is printed on.

Well, I just looked on HOP2, for ORD to MXP on Feb 14 and returning Feb 28. HOP2 quoted $677.21 with Swiss. Same flights on Swiss site quoted $1640.00.
At that price I'd book it, and if it was phony, dispute the charges.

On Google flights I see $963 on SAS via Copenhagen and $723 on Aer Lingus/BA--but those involve much longer layovers. Swiss and Lufthansa have much faster connections and are very high.

BUT: Both Lufthansa and Swiss are bookable on Kayak for under $800. I've never booked through Kayak, but it has been around a long time, and I'd be much more comfortable with it than with Hop2. [Edited to correct incorrect airline above. It's Lufthansa and Swiss that are under $800 on Kayak (not SAS and Swiss.)]

It looks to me like there's not a lot of demand for those February flights, and the airlines are making the tickets available rather cheaply to consolidators while trying to get full price (and then some!) from direct purchasers.

I made mistake of booking through Hop2 at very good price, too good to pass up. We called the airlines directly and they said they could not offer the same price so we went with the cheaper fare at Hop2.

Tickets issued fine, but 3 different airlines, 3 different check-ins, and many problems with double ticketing, paid seats not connecting to the airlines systems. Also one of the airlines has already changed my return flight times twice making a very tight connection transfer time.

I am now frustrated and very nervous about actual flights will even happen. I will return to my personal policy to book directly with the airlines to avoid all this in future. Not always worth the savings.

DO NOT. I repeat, DO NOT, purchase from these people.... it IS too good to be true. We purchased r/t air from LAX to London through Hop2. Our return flight from Paris (Paris to London - included in our purchase) was cancelled and they nor the airline notified us, so we were stuck! Missed our London to LAX return flight and were shoehorned into a nightmare Delta flight through Atlanta! Nightmare flight and not what we paid for. When I tried to get answers, by email, phone and chat - chat was the only way, they didn't return my emails or phone calls - they just blamed the airline... my receipt shows I paid them for travel, but they could not answer or provide any solutions on our behalf. Don't book through these people... the price is tempting and they are trying to pass off as legit.... there's a reason why Expedia or any of the other big fish have not swallowed these minnows. They're toxic! Buyer beware.