Should I fix it and sell it or sell it as is? Mark Levinson No. 331

 Why would someone pay $800 for your amp if it needs a $1,100 repair? How much would it be worth if you have the repair done, and then sell it in good condition?

It most likely needs one or more power supply caps replsced. Not expensive! If you are handy with a soldering iron you can do it yourself. 

    Even if you pay the $1,100 to have your ML 331 repaired, it still makes more economic sense to do so than sell it as-is for $800.  You'd only need to sell the repaired 331 for $1,900 to equal a net $800 and I believe you could sell it for at least $2,300 if you advertise it as having a recent ML authorized dealer service checkup with new power supply caps installed with the documentation to prove it.
     I also think you'll find it's much easier to sell an amp that's in perfect working order than one that is not.  Who knows? You may even decide to keep it if you request upgraded caps.

Tim