I have a few sites that I am considering accepting guest posts on (for a fee or perhaps for a barter).
What is good criteria for accepting a post?
How useful are posts from respected agencies as quality, additional site content?
I usually tell people, don't send me something if you wouldn't publish it on your own site, in fact I go one further, only send me a guest post if you are already debating if you should publish it on your own site!
Be strict and only accept high quality. If you do write something about guest posts, you will be inundated with offers. 90% of it will be pure garbage, 9% will be ok'ish and 1% will be really good, just so you know what to expect.
I don't mind accepting less than stellar guest posts, but only if I have an agreement in place already to cross post on their site(s).