Unbelievably low fares on this site. So low that “buyer beware” sign flashed in my head. Read an old post on Fodors that they sell other people’s tickets that they bought (with cash or miles) that they can’t use. Is this true? I don’t want to get to airport in France and discover we have illegal tickets that can’t be used.
I've heard of them (maybe from this forum), but based on your description, I'd steer clear of them.
There's a legit site called "Seat Guru" that has very useful information about the seating on various aircraft. There's obviously potential for confusion between that website and Flight Guru. I don't remember any discussion of Flight Guru on this website, but from your description I would avoid it.
However, I see internet references to both "The Flights Guru" and "Alpha Flight Guru" and don't know which one of those you've encountered.
Any agency offering significantly lower ticket prices than the airline's own site should be scrutinized through very suspicious eyes. The days of last-minute or bulk-buy discounts mostly belong to history. If you proceed further through the buying process you might find hidden charges or fees. The tickets may be sold in a package with hotels, car rentals etc. (although those packages may be worthwhile if you get what you want rather than what the vendor wants.) Resold points tickets enter a minefield.
Buying straight from the airline, with no intermediary taking a slice, is the safest approach and can ease backup if needed.