Same experience as the others. These people are out-and-out thieves.
I received a quote from them on October 14 that I would be paid $90 for my iPhone 5 assuming they received it from me in good condition. They sent me a mailer and I sent my phone to them on October 23. On October 26 I received confirmation in writing that the phone was received in good condition and I would be paid within 1-3 days. I never was. Beginning November 1 I sent them numerous emails which were all blown off with form responses or ignored altogether. The only phone number I could find was disconnected. After doing research to see if others had had this problem I was amazed to find the scale at which this has been an issue; the BBB has received dozens of complaints and made a formal complaint to the company, which ignored it (http://www.bbb.org/new-york-city/business-reviews/recycling-computers-and-other-electronics/guzu-inc-in-deer-park-ny-129989/). They say at that link,
"BBB files indicate a pattern of complaints concerning problems with mobile device trade-in offers from Guzu. ...However, as several weeks go by, numerous consumers reportedly still haven't received the cash for their devices. A few consumers who have received a check from Guzu claim that the check has bounced and they are charged a fee for that by their banks.
On August 12, 2015, BBB requested Guzu's voluntary cooperation in eliminating the identified pattern of complaints. As of September 11, 2015, the business has not responded to BBB's communications about this matter."
There are dozens of similar websites compiling exactly this same pattern of fraudulent behavior (e.g. http://www.complaintsboard.com/?search=guzu). While I am of course biased since they ripped me off personally, it does seem like this is a genuine, recurring issue and in fact I have not been able to find a single real person who was paid by them other than the "testimonials" on their website which I suspect are also made up.