Usedphones.com Usedphones will ISSUE you a PO, but they will never PAY after you SEND your equipment, Internet

I mailed equipment to Usedphones.com in January, after receiving a PO from their on-line website.  According to UPS records, it was received by Usedphones.com on January 30, 2012.  According to Diaz, their customer service representative, the equipment was accounted for on February 2, 2012.  Since then I have made repeated calls to Usedphones.com inquiring about getting paid for the phones, that we, in good faith, mailed to Usedphones.com.
 
On February 23, I called and was told, the check is on the owners desk waiting for signature. Again on March 15, I called and was told, the check is on the owners desk waiting for signature.  The owner of my company called on March 21 and was told, the check is on the owners desk waiting for signature.  The owner just AGAIN called Usedphones.com and was given the same run around.the girl responsible is out of the officeblah, blah, blah, but the check is on the owners desk waiting for signature.
 
According to ther terms, payment should be made 30 Working Days from receipt of equipment and March 19 was 30 working days. After sending the above to every email address that I could find on their website, I did get promised a check the week of March 29.  As of today, April 10, I have gotten no check, just a bunch of empty promises.