I would never buy an aftermarket shaft for a cue until I had the butt in my possession and knew more about the pin the cuemaker used. Some cuemakers play around with different pins and since a lot of cuemakers turn their own pins, they have their own bastardized pin geometry that might not fall in line with a UTS or ANSI standard thread.
That being said, not all pins of the same diameter and thread count are the same. Some pins may have different minor diameters, pitch, etc. so that a 3/8-10 shaft from X cuemaker might not fit on an aftermarket 3/8-10 shaft from Y cuemaker.
Based on that, if the seller correctly represented the cue and you bought an aftermarket shaft with the correct pin size and count and it didn't fit, then you are on the hook for the mistake in not doing your due diligence in determining the correct pin. Since that is not the case here, both of you are really at fault. If I was the seller I would split the cost to return the shaft for a new one or split the cost to get it tapped to the correct pin geometry and end all of the drama there. Both need to chalk it up to a lesson learned.
Kelly