That thing is a beast and would suck to carry a mile to the deer stand....doesn't t mean I'm not going to do it (I will definitely do it), just stating it will suck.
The MRAD is in great condition. The store owner said the previous owner put 40 rounds through the 338LM barrel and never shot the 308 barrel.
As you mentioned, your decision could be based on infatuation as opposed to clarity of mind when considering overall utility.
The rifles you mentioned to trade all have a niche for YOU for whatever kind of hunting/shooting YOU do so you need to factor in if the MRAD will satisfy that "itch" for shooting Prairie Poodles or would it be an enormous case of overkill when compared to a lesser caliber(s).
Then there is the cost of each round fired assuming the MRAD would become your Do All to fulfill all the hunting/shooting you'd do with the lesser calibers.
Even reloading, with the larger caliber of the two barrels, sending rounds downrange regardless the target will be high while the .308, not so much. Especially when factoring in buying NEW reloading components and/or equipment if required.
So it is ALL subjective and an itch only YOU can satisfy by making the trade, or not.
And that isn't taking into consideration the actual $$$ value of the swap, either.
As others have said, it is a decision that YOU will have to live with and only YOU can decide if the trade would make both financial sense as well as satisfy the desire for an exotic (and totally BITCHIN'!) piece of new hardware to show off at the range.
My last and only caveat would be to insist on an opportunity to shoot BOTH barrels at the range (your cost for ammo, Natch) to ensure that bad boy CAN hunt.
I'm sure your LGS Dealer wouldn't intentionally take advantage of you but rather than end up in a contentious battle because the rifle is a lemon, (and learn that is why the previous owner divested himself of it) better to have ALL the factors covered and specified up front.
All that said, let us know your decision and send a picture or two of your new acquisition should it happen for you.
Good Luck, God Bless, and.....
Overnout