Hi. I'd say it depends on how confident you feel. On the other hand, you could weigh up $650 of risk and decide from there.
How much can you ascertain the "mint condition", as seen, or as described?

I know they're (somewhat) different cameras, and have you done a comparison of the two to help you decide.
Here are two links to DPReview:

EM10-Mk2 (and handy comparison table) http://www.dpreview.com/articles/488...-m10-ii-review

EM-5: http://www.dpreview.com/reviews/olympusem5

I note that the EM10 is not weather sealed, wheras the EM5-11 (dunno about mk-1) is weather sealed.