No one can make a business out of buying old smart phones.
If they can turn around and sell them, then they might have something.
But, only about 300 people per month search this exact term. And the CPC is low.
Goog it yourself and you'll find most results are about finding uses for old smartphones.
People that want to buy one are probably using some other search phrase.
End user: $RegFee to low $xxx.
Reseller: $x.
Maybe in 10-20 years the domain value will increase, when we can talk about old smart phones.