I personally have had success with the Google Keyword Finder. This is part of the Google Adwords program for estimating keyword traffic and since Google has statistics on everyone's surfing and search habits, this is your best bet.

My advice to you is that before you try estimating server traffic for your marketplace, select 5 to 10 short and long tail keywords that you want to target and then use Google's Keyword Finder to look at the search traffic.

I hope this helps.
Bruce