to add to what franklin said, in regards to if they pay and what you do if they don't. and also just a disclaimer first IANAL(I am not a Lawyer).
with that out of the way, if i was going to consider accepting a P.O. from a company, the first thing i would do is do a google search, see what info there is on them, how long have they been in buisness, what type of company are they(Partnership, LLC, Corporation ect.). And if you are really concerrned you could consider looking up what info Dun & Bradstreet Credibility Corp(http://www.dnb.com/
) has on the company(they are basically the equivalent of Transunion, eqafax, experian and the like, but for business) I'm not sure what D & B will give you for free, i think you have to pay to find out very much on the company.
And regarding non-payment, depending on the exact wording of the P.O. there may be a specific clause in it regarding YOUR recourse for non-payment, otherwise, I believe the method of recourse would to take them to small claims court. The biggest thing is don't go giving a company you have done business with the ability to send you a P.O. for thousands of dollars, limit how much they can spend until you have built a relationship, then you can raise their "limit"
*****Disclaimer****** I am not a lawyer, the above should not be taken as legal advice, if you need to know 100% fact, please consult a licensed legal professional. I have taken a business law course(last semester) that is what I am basing most of what I have said on