Sell them on Craigslist or undervolt and use them.
What do you mean "undervolt and use them?"
Undervolting is where you attempt to reduce voltage/clock speed and current of the miners.
Don't do this, it is rather useless, otherwise you would just run them at a normal speed.
That advice doesn't seem too useful.
Undervolting and underclocking can reduce the power consumption quite a lot. It may increase the efficiency by 40%, but the hash rate will also reduce.