It sure appears to be a #3 since it has no firing pin retaining screw, and has the 3 line rollstamp. An early JM Marlin #3 that is a flat top receiver instead of the later concave top.
My bet is it's a rimfire, and possibly been rebored to .25 caliber. Unless the seller is just plain mistaken on the bore diameter and it's simply the typical #3 in .22 Long.
If he tried a .22LT cartridge in the chamber it will answer the question. And maybe remove the forearm to see if there are any markings to reveal what chamber or bore it has if the person or company doing it marked it? 
Stevens did this type of work, and always marked their work. My #3 done to .25 Stevens RF is well marked under the forearm.
Here's the later concave top #3 on the right:
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My #3 in .25 Stevens:
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Breech

Sure look bigger than 22, probably bored toa 32 RF. I'm going to pass if I can't reload for it. Thanks everyone for your help!...LK