Would you buy a piece of gear with a defect?

if it's just one input one channel, its likely something bad between the rca jack and the switch, and likely easily fixed.  I'd worry a lot more if one channel didn't work on all inputs.

Any competent tech could diagnose and most probably fix it.

If it were me, I'd go for it, especially if I didn't need all inputs immediately.  

Have you looked inside? JC2 Manual Online (htguide.com) I'm guessing you don't have a schematic. Preamps are usually an easier fix than other components. I read something that this is an FET preamp, if so I'd be concerned about obsolete parts. Do a lot of research! I would offer less than George. Good thing is you have a channel that works, so if it's an open trace on a multilayer board, time and patience could serve you well.

If you have the relevant DIY skills, maybe yes.

Although only 400 off a used comp for a unit  non cosmetic damage seems high to me as others have noted.
(Cosmetic damage is another matter, and I'm I'm happy to buy discounted items where that's the only issue)

If you are not DIY, note that finding a competent tech can be difficult, and some problems will vex even competent techs.  And in any case it won't be free, which is likely to eat up a good hunk of any savings.
Early in my time in the hobby I bought an Adcom GFA-555 from a friend.  It sounded like they sound, but had a buzz (amusingly, his cat puked on it, and gunked up one of the boards, through the vents). 

I took it to the only local tech I could find (in St. Louis, a pretty large metro).  Twice.  He claimed falsely to have fixed it, and was abusive to me in front of other customers on the return visit, when he denied hearing the fault.
I then, with a bunch of hassle, got the schematic, and a friend who is very smart electrical engineer went through it.  He couldn't fix it either.
So ton's of headache, and no cigar.
If you are not DIY adept with electronics, I'd pass. 

I do,  I normally look at what the repair would cost from a reputable shop, then offer according to about 15 to 20 percent below used market value.  Then I do the repair myself. 
In my opinion to buy something and go through all of the time and trouble to get it repaired and not come out ahead doesn't make much sense, unless of course, you are obsessed with a certain piece or they are just near impossible to get.   Good Luck 

Depends on why you are buying or selling the component. I am currently selling an non working Sony SCD-1 for parts as it appears the laser is dead and parts are unavailable. I have also tried to purchase a working unit to fix mine but not willing to pay the prices folks are asking. So if I can sell mine for parts to someone to help them out then we both win. 

Chuck