Large mice just aren't going to cut it for a snake that size. I strongly suggest medium rats at the very least.
Oh, and it's really fairly normal that it wouldn't eat after changing hands. They're sensitive that way, but once it's settled in and eating for you, if you dramatically increase the size of the food you're offering it, the thing should be OK and it'll likely pull itself together quite quickly, but don't be surprised if it doesn't eat all that often, which makes it even more important that it has proper sized meals when it
does it for ya