Would it be possible to add a lens "tagging" option for the marketplace? When you create a for sale thread, you would be presented with a list of lens. You would then select one (or more if you're selling multiple lenses) and the thread would be tagged with that lens.
Buyers can then choose from a list of lenses and only the threads that have that lens would show up.
I know you can search, but sometimes people type the lens name differently and it might not come up in the search results