Altamaha is correct. To sell this to any builder there is much that needs to be shown.
To simply screw in a chambered barrel is not enough and in some cases it can be worth less then a blank barrel and military receiver.

To make a good magnum from a Mauser the work needs to be done perfectly.

You need to open up (or replace) the magazine to hold the cartridges perfectly.
Next you need to open up the bolt fact to fit the shell and fit the extractor.
Then you need to install, chamber and headspace the barrel.

When that is done you need to modify the feed lips to feed the cartridges perfectly (That's a lot of work)

If the bolt face is not done right you have problems. If overcut you need a new bolt.

If the head spacing is not done right you have problems. Too tight and you'd you need a reamer. Too loose you need to set the barrel back, which requires a lathe.

If the chamber is not cut smoothly you have problems. Again you need a lathe and the reamer to repair this.

The rifle needs to come with dies and shellholder and/or ammo for you to even be able to check for these problems.

So your price of $600 is very good if all the work is done perfectly.

If the box is opened up and the gun feeds perfectly, if there is a bit of ammo to prove it, if you have a test fired shell or 2 to show the chamber is cut right, and it's all perfect, $600 would be a low price.

If there are any problems listed above the gunsmithing to fix them would cost at least $250 and could be up to $550, so as you can see you may have a $600 (or even much more)) piece of merchandise there, or you may have MUCH less.

Pictures and information is what is required for you to sell what you have.

I hope this information is helpful to you.
Good luck