This is my biggest problem as well... There is no search function for items as of yet, but apparently a search feature is in the works.
You're best bet, after this week's patch is to go to the categorized Market Wards, and if people understand how that system works, they will be selling the right items in the right ward...
Unfortunately, from what I've seen it seems people will still just pack in the first market ward, but some people have gotten an understanding of the new system and are using it properly, sending their retainer to the most fitting market ward category.
I for one don't even bother going in the first market ward anymore... It's too laggy and packed with stuff that has no relation to it's category.