I fidn it helps to think a bit sidewise as well. So, if you're after a Specialized Rockhopper, for example, don't search on that. Search on just rock hopper, rock or even just 'bike' and narrow things down by value / location. Quite a lot of the people who list on eBay aren't that literate.
Oh, and use the 'search in description' check box as well... I'm shutting up now... they're mine, all mine... mwuha ha ha ha ha
