I've been trained by lots of reading and listening to Sherpa reviews to mostly stay away from number domains in contexts where they replace words (4 : for, 2 : too, to). However if it's a strong term then there most certainly could still be value.
On that part however .. I can't be 100% certain .. but I'm guessing that 2nd hand phones still are a pretty big market. Although I'm not if it's actually strong enough to get a high multiple on purchase price.
Working against the name is the non-obviously redundant "smart", as people looking to sell their phones would likely just use the term phone. While the "smart" isn't even that bad .. it does take the domain to 16 characters. Then also Cash could be replaced with money.
But still .. all in all .. could be a good domain .. just not sure what people selling 2nd hand phones would be willing to pay for a domain?
I think the saving grace for this domain is that there likely is far more 2nd hand phone resales than most would guess.