I think Alexa is a great tool. I use the Alexa top sites API every month to find growing internet trends. I wouldn't buy a traffic domain unless it had an Alexa rank.
Check out www.hitgeist.com - the data is a bit old, but great for finding new trends through gainers in traffic.
Thing is though, using the API, the $$$ do clock up after downloading lists of millions of sites then comparing data via the Alexa web information service (AWIS) to create custom lists of new gainers in traffic.