how much you want for your gunsmith opinioned "junk" upper?

2-5 tops mfg of the actual upper. any ar can be fixed, unless there is excessive headspace (closes on a field guage) the bbl has been shot to sh^&*T or bent.
as for the rear sight you need to knock the cross pin out (keep an eye on the spring) take it off check the ball detent & spring in the l/side of the sight. it might have a spring that's too long putting pressure on one side cocking it.or to short allowing it to flop around. when you reassemble you need to compress the spring inside the hole of the rear sight so you can get the pin back in. i dremeled a notch into a crappy screw driver for that custom gunsmith tool