Pard, I would agree that that price is too much, especially for a gun that does not work. I would only pay that if the rest of the gun was in exceptional condition (lots of finish, perfect bore, good wood, etc.) I have some experience working on Spencer pump guns (you can see some of my posts on this forum) and while I can’t say definitively, I have not seen this issue on other guns.
If the gun cycles properly, so the block goes up and down and a shell would cycle from the mag, to the chamber, then get ejected, But the hammer does not stay cocked, then I would suspect a problem with the sear not engaging. This could be the result of a broken seat on the hammer or trigger, there could be replacement parts that are not allowing the hammer to go back far enough to engage the sear, or there may be a broken/missing trigger spring. The latter is an easy fix, the former two issues are a bit tougher. You can test this by pushing in the trigger while racking the gun to force it to engage. You can actually see the sear inside the trigger guard if you hold the gun upside down, so seeing what is happening and what the condition of the sear is pretty easy.
If the sear engages but the hammer is not moving forward when you pull the trigger and just flops around, then you have a mainspring problem. The mainspring in a Spencer is a brute if a “V” spring. One leg is attached to the block and causes it to spring up to eject the shell. The other end engages a stirrup on the hammer. It’s kind of a pain to get in place so it’s possible that someone assembled it wrong, or the spring is broken. making or finding a new spring could be tough. Assembling it properly could be easy. One way to tell if there is an issue is if the bolt does not slam up with authority. The bolt will move up if the hammer leg of the spring is not properly engaged, but with less force than it should.
Happy to help you diagnose things if you can talk them off the ledge on the price. Personally, I have paid about $350 for both of my Bannermans. I have not run across a real Spencer for sale.