Sachel,
Sure the rifle has no collector value, but I'm betting that you are not buying it as a collector piece, and just want an interesting shooter.
If that is the case, and there has been no modification to the action, barrel or any other metal, and the action has not been drilled and tapped, and all of the metal is in very good condition, then I don't think it is too horrible of an asking price.
The scope is worth $100, so basically they are asking $650 for an Inland M-1 Carbine with a sporter stock. You could buy a standard replacement stock for what, $125? And then you would have paid $775 for an Inland M-1 Carbine. Not a bad price, considering that the asking price for M-1 carbines with mismatched parts and numbers is now about $950 at every gun show I visit.
And remember, the $750 is just the asking price. I'm betting you could get it for $650. A rifle like that could sit on the store's rack for a long long time. This would reduce your price for conversion to a standard M-1 to just $675.
On the other hand, if the action has been drilled and tapped, then this comes under the category of "Somebody bought a $950 gun, spent $250 on custom modifications, and ended up with a $400 gun." I just wouldn't pay more than $400 for it. And, if you ever need to sell it, it could take a long long time.
The .30 carbine round is such that you really can't get much more power etc. by reloading it. So, check the standard ballistics on it, and realize that this is what you would end up with. Personally, I think the .30 carbine would be fine for varmints, out to 125 yards or so, as long as you are not trying to shoot prarie dogs. (Probably doesn't shoot flat enough for varmints that are that small.)
Regards,
Mannyrock