I'm not sure I would require a POF but I would require some proof that they're not going to waste my time.
There is other ways to see if they're qualified and capable of buying homes from you. Find out what other properties they've purchased. Are they working on a current project now? google their name, company name etc.... check the county records to see if and how many deeds have been recorded in their name. I know this seems like a lot but it's really not bad.
I don't have a problem telling a buyer/wannabe buyer to pound sand just as quickly as Jon would laugh at someone but I know Jon actually does buy and rehabs houses so I wouldn't bother asking him anything other than what he wants.
Also, I second Ryan's reply, there a million different posts regarding this and other very closely related topics. I would highly recommend using the search function as you'll get a lot of answers to questions you haven't even thought of just by reading what's already been discussed.