While the action can certainly handle it, there's no way to tell the headspace's correct w/o checking.
Since the 6.5 Rem Mag is a belted magnum, and the -06 is not, the bolt face would also most likely have had to have been "opened up" so the larger case head would fit it.
God only knows how the differently-shaped magnum cartridges feed from the magazine - which depends upon how/who reworked the feed surfaces built into the action underside @ the magazine well.
In short, until it's tried at different feed rates, you may/may not have a single-shot rifle.
Of course, the belted magnum will exert more recoil forces on the stock, so I'd also want to ensure the bedding was correct, to preclude any stock-splitting in it's future.
I'm not saying it's to be avoided, just that these things should be considered when calculating how much you're willing to pay for it - Now, and/or later.
A written guarantee, from a retail store/FFL, is different from some aquaintance saying "It'll be OK".
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