I would suggest reading the avast Help File under Silent Installation, it has all the switches, etc. This function would however, appear to be only available on the Pro version.
The only query would be which version are you intending to install? If as I suspect it is the free avast 4 Home version, how do you ensure that the machines being sold are going to people who qualify for the free version, e.g non commercial use?
I can remember something similar being asked in the past, a search of the forums may prove useful. I'm sure that one of the Alwil team can point you in the right direction about pre-installation on PCs you sell.
HTH David