[the salesman knows I like 22 Mag.s,and thats why pointed out to me ,he said few where made]
He is trying to lead you down the garden path, to make a few bucks
[I found one in local shop 4in. barrel SS looks new ..is it worth $350.00 ?]
It would be worth well over $350, if it was in the original L.R. chambering - and not rechambered to .22 Mag.
Ruger only chambered the SP-101 .22's in L.R. - not Magnum.
It only has about a $250-300 "shooter" value, if you want to take a chance on the accuracy.
Also, the SP-101 has a windage-only adjustable rear sight, and most likely the mag rechamber & use on mag ammo will change the POI beyond the ability of the sights to compensate.
As you may already know, the mag chamber makes it dangerosly oversize for the firing of L.R. ammo - which could lead to burst and certainly blown out cases/spitters.