I'm wondering if it is ok to use almost the same meta description on every page. I'll use my classifieds site for example. Let's say that you make the meta description on an ad to be: <ad title here> on the free gear classifieds. So if someone posts an ad with the title "Gibson Guitar for sale", that means the meta description will be: Gibson Guitar for sale on the free gear classifieds. Just like that with every ad... Is this ok? Or not so good?
Well its he best way.. try to unique.. if contains duplicate then will get error warning of meta duplication from Google
Not advisable in terms of SEO. They should be separate for all pages or not at all. I had invited penalty from google for duplicate meta descrip once.