Before buying it, I would check a couple of reference books on used gun prices to see what a good price range is for the item you describe. Of course, anything is worth what people are willing to pay for it, not what some book says, but a book reference will give you a place to start when negotiating.
A WWII vintage GM in 38 Super seems (to me) to be more of a collector's item than a shooter, but what you do with it is entirely up to you. I know several people who shoot the heck out of vintage pistols. It is really your call.
As for the 38 Super cartridge, it is still commercially manufactured by UMC (Remington), Winchester (in ball and silvertip), PMC, and a few others. Prices range from $10 a box and on up. It is an under-appreciated cartridge that deserves more attention.
True, it was popular among IPSC shooters about ten years ago, but I haven't kept up with that game, so I can't say now where it stands with them. The 38 Super is a good cartridge for the handloader because it lends itself to many loading options. I would stay away though from trying to "make major" with it, especially in an older pistol. It can be a good performer without too much hotrodding.
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It only takes one bullet to change your life.