In case anyone is new to multimining, I recommend using an exchange like Cryptsy which has auto-trading.
Put your Cryptsy deposit address for each alt-coin (eg. PPC, TRC etc) in as the multipool.us withdrawal address, and on Cryptsy set the coins to auto-sell. This way you'll still just get BTC from your mining, but a little bit more when the alts are more profitable to mine.
What's the average profit that user make with autosales on?
It varies from day to day, depending on altcoin prices. You can make 150-200% of what you would make mining bitcoin on a good day.