We all make mistakes buying guns; this was yours. If you can break even, fine, but I'd take whatever I could get for it, & move on.
When you check Gunbroker, look at COMPLETED auctions, not what folks are asking, which may not be realistic. COMPLETED auctions tell you what someone actually bid.
Yes, folks sometimes get $500-600 for a CAI M1rifle, but only from someone who is new to M1 rifles. Those who know about them will not buy a CAI, at any price.
Twice I assisted shooters at my range who were having trouble with newly purchased CAI's. I field stripped both rifles, so I had a chance to closely examine all parts. Not only were the receivers out of spec, but the parts were well used. I was able to get 1 rifle to function, but it could only be taken apart/assembled with a hammer & drift punches. The other rifle had a bad op rod slot, & required welding/gunsmithing. In both cases, I warned the owners that their rifles were not safe to fire. I didn't do any metallurgical testing, but I don't think anyone should take a chance on a rifle that they know is not made right.