(MONEY Magazine) – Want to fly round trip from Washington, D.C. to Rome for $500? It would cost $800 to $1,229 on TWA. Or how about New York to Paris for just $380, instead of Pan Am's ''bargain'' $548? Tempting, huh? Welcome to the world of consolidators, or air travel discounters. Operating legally in the U.S. since airline deregulation in 1978, about 100 consolidators can be found across the country (see the box for the best). Most specialize in foreign travel, but some also sell domestic tickets. Consolidators are still battling a bad rap left over from the days of fly- by-night companies and tickets on Mystery Airlines. Yet in New York City, home of most consolidators, the state attorney general's office has found no grounds to investigate the industry. And New York City Department of Consumer Affairs' Patricia Cohen reports: ''We have found only mild complaints in the | past three years.'' Here's how consolidators work: when airlines can't fill seats -- like now -- they sell them to a consolidator at a big discount. In turn, the consolidator offers them at about 20% to 50% below normal prices to travel agents, other consolidators or directly to you through newspaper ads. Discount tickets, like all bargains, require some trade-offs, says Clifton Cooke, publisher of Jax Fax, a discount-fare trade magazine. Usually, the tickets are nonrefundable and cannot be changed. And if the flight's delayed, you probably won't rate meals or other compensation. ''It's logical that you give up something,'' explains Cooke. To avoid surprises, he says, ask for policies in writing. Some companies put your payment into an escrow account until departure, which protects you should the consolidator or airline go under. Perhaps the biggest hitch is the consolidators' widespread reluctance to name their airlines, which makes people nervous. Most consolidators simply promise ''a major scheduled carrier,'' because, says David Kols of the St. Louis-based UniTravel, ''75% of our contracts state that the name of the carrier must not be released until ticketing.'' Airlines fear the information could undermine their fare structures. Smaller carriers don't impose the same restrictions, and many need to fill seats between U.S. cities and European stopovers en route to home countries. Your New York to London Air India flight, for instance, may go on to Bombay. If you're uncomfortable dealing with consolidators, find a travel agent. Most consolidator tickets, says Phil Davidoff of the American Society of Travel Agents, are sold by agents. Tips for shopping: -- Read the fine print of consolidator ads -- ''OW'' fares, for example, are often one way but based on a round-trip purchase. -- Get the ticket as soon as possible, perhaps by overnight mail. That way, you'll have time to correct any scheduling errors. Also, make sure there are no alterations (flight times or dates covered with revalidation stickers, for instance). -- Ask about add-ons. Weekend travel may cost $25 extra each way. And don't forget the standard $26 departure tax on international flights and, lately, the fuel surcharge of $20 each way. -- Pay by plastic -- even if it's more expensive -- so you can cancel payment if problems aren't corrected. If the firm refuses cards, you're probably better off elsewhere.