Accept them and move on. No real need to reciprocate.
+1. I have a ton of endorsements (some for crazy skills I don't even have, like speechwriting!) but I never give them. I think they are cheesy and ridiculous.
Sometimes when you sign in to LinkedIn, it will show a pop-up box that says 'do you want to endorse these 3 people for these 5 skills' or whatever. I think people click yes just to get to what they wanted to search for on LinkedIn without actually realizing the endorsements they are giving. So keep that in mind too.