The two rifles (Kar.98a and Gewehr98 - 98k too, though originally the Imperial rifles were designed for S patrone, they will all shoot sS patrone fine, most interwar rifles will have a rearsight designed for the sS patrone anyway) use the same ammunition, the bolts are interchangeable, though have different handles. The Gewehr98 has a straight handle, the 98a a curved bend that is different than the 98k, and has a flattened handle knob that is checkered.
CB's comments relate to the unit markings and the associated interwar service the Kar.98a is known for (police and training). Generally, look at the buttplate, top tang usually but can be on the main part of the buttplate, here there is sometimes unit markings, they can influence value significantly if the rifle is original. The take down also sometimes has a unit marking, though it will be an interwar unit marking and often police or training related. Stocks can have markings too, HH recently had a "G.A.L." marked 98a, not sure what it represents but it is typical of the possibilities, which seem endless, - PwB, EWB, REV are known on stocks, RFV and DR are known on the buttplates, the purpose of these are known, but other possibilities exist and the interwar Gewehr98's have several other organizational markings not yet seen on Kar.98a.
The $375 98a sounds like a bargain as described, though only an examination of the rifle will tell. Interwar rifles are generally not as valuable as Imperial rifles, though interwar rifles that are 100% matching are rather rare, ones with rare or unusual markings can be worth significantly more to the right collector but matching is more important when it comes to a rifle having any value. (simple fact is a mismatched interwar rifle, even with nice markings, will generally be worth much less than a matching rifle and often less than $300 to most collectors. I wouldn't buy a interwar rifle that the stock is mismatched, they really aren't worth much and are hard to flip - usually they are worth more parted out, usually much more...)