These 3rd parties are called ticket aggregators.

Some will just work on a commission basis, and discount the ticket to the buyer by sacrificing part of their commission.

Some will negotiate with airlines for lower prices when they buy tickets en block for resale, and pass some of that savings onto their customers. In this scenario they'll discount heavily as the date of the flight nears. Better to sell cheaper than to be stuck with unsold tickets. Several times I've snagged prices way lower than at the airline website... is good for casual impromptu travel.