There are two ethical considerations. One is the bait and switch aspect. To me, Travelocity failed this test, while the others passed. Orbitz and CheapTickets, for example, explicitly state when there are only a couple of tickets left on a flight. The other is the protection of consumer information.
A found that all four of these sites did a good job of protecting my information. They all appear to have strong software and have built in steps to ensure confidentiality. I did not see any reason to feel that the confidentiality of my information was threatened. I would make payments on all four sites.
A find that for the most part all of these websites would make me less likely to do business with a travel agency. My travel needs are mainly transportation. Each of these companies provides multiple options that are easy to compare with one another.
They also offer deals on ancillary products and services that can help negate the need for a travel agent. Where all of these sites lack is comprehensive information on key destinations. They seem willing to offer hotels, car rentals, event tickets or anything else commercial but they leave out basic information, prospective itineraries, best travel times and other such vital information. They lack information on visas, immunizations, and other logistical issues. They also fail to provide insight into airline ticket pricing strategy. For the most part, each of these sites appears to function as a reseller, rather than an all-round travel provider. They work for travelers who are willing to do most of the homework on their own, but fail to meet the needs of travelers who seek additional information beyond those features that can be sold..