Buying used, US to Canada, tax and duty?

I suppose they would charge duty + GST but on what amount?
The duty guys can't tell the value of the parcell unless there's a sales slip on it can he?

If the sellers says he sold it for 5.00$ plus shipoping, how could you get charged on a higher amount?

USPS as in Us postal services? I'm asking 'cause i'm about to buy a preamp across the border. Is that the way to go to avoid beeing ripped off?

Thanks