I traded in a device for credit, and upon ID verification there was an issue of needing my SSN which I declined to provide. This has never been an issue with any other site/vendor that I have worked with, even given it being a stated parameter of the trade. There could have been something done, or just actually cared enough about their clientele to actually help. After many back-and-forth messages with a 'customer service' tech that was predominately not willing to resolve the issue, I requested to speak to her manager.
After receiving the call, the manager was also unwilling to offer a suitable alternative to providing my SSN and/or a solution in the least, and I forfeited my credit AND system - having never received either.
To say that Back Market is quite possibly THE worst service I've ever received would be an understatement. Absolute garbage.
I would NOT recommend them to anyone. You're better off playing more elsewhere.