There are a few urban legends about "Market Dave".
I think his real name is something like David Oakel, which is quite a good name for a local yokel.
The key legend is that he is very rich, "but doesn't like to show it". Well he certainly hides it well. He never seems to spend much cash and has had the same clothes since the 1960s, so there could be something in that.
He sometimes drinks in the Malt Shovel, where he was once reported to be having an in-depth conversation with our dear Kenny Palin. That would have been awesome to overhear.
As to his never ageing and immortality? Well he certainly has no fear when facing down on-coming traffic with his cart. Perhaps the outdoor life and exercise had rendered him lean and fit or maybe he has an ageing picture of himself stashed in his attic that does his ageing for him (like Dorian Gray).