Found a good price boston usa to bkk return for 500 less that the airline itself. Is this too good to be true. What can you tell me about 1800airdeal.com

500 what? Dollars or Baht?

Just did a dummy booking with them, but the other way round

from Bangkok to London (for my case) and found them to be more

expensive than direct from the Airlines.

I would suggest

www.amadeus.net

Or this one that searches a number of websites and gives you the cheapest supplier, sometimes.

http://www.dohop.com/