Strictly speaking, this isn't a Support issue (the Support forum is for technical problems with your wordpress.com blog) But I'll answer as best I can:
A repost is different from guest blogging. Guest blogging is where you write a new and usually original post on someone else's blog with a link back to your own. The idea is that that person's readers then go to your blog, and yours go to the host blog - it's a way to get some publicity.
Reblogging takes a small part of a post - usually no more than the first paragraph or two, plus one large image and the rest as thumbnails - and it automatically posts to another blog.
You can find out more about reblogging WordPress.com posts here:
As for the etiquette of guest posts, the usual thing is to answer all or most of the comments on the blog on which you're guesting. Though if you don't want to, you should say so or at least find out from the host blog what is expected of you.