FK is 100% correct on the pressure levels. the CUP numbers have NEVER been cross compared, to the PSI numbers. They are two different worlds, and measured two totally different ways. CUP actually measures the crushing of a copper pellet in a special chamber. The PSI is just that, PSI measured in the chamber, and outside the chamber. When someone ever publishes a chart that directly compares CUP to PSI and it is verifiable, then we no longer have to guess.
I would not risk my eyes or face, on the word of someone that "knows" something. If Marlin ever published a statement that the 336 action is strong enough to handle those cartridges, then I would do it. Not me. I doubt they will because I'd bet older actions are not as strong and they know it. Flame me if you like, but There's a reason the 307 never went to market, and the others were dropped. I do not think it was ALL about sales either. Not flaming the Marlin, at all. I love my 1895SS and I doubt I will ever part with it, and I wish I could scope my 375, but being a top eject, and a family heirloom, I will not modify it to make that possible.
As for the price, that looks like a fair price, not a steal, but fair, if it is in very good condition. I remember I passed on a 307 a few years back, for $400, when I was looking for a 356, to go with my 375BB. Big mistake on my part.
Unless you see one at a better price, I'd get it. Besides, if you pay too much, at least you have the rifle, better than holding off, and never seeing another one offered.