I am struggling to understand the use of 'a', 'the' and zero article (see italics in bold) in this example:
Goods are consigned to a factor for sale. He makes advances, in consideration of an agreement by the principal that his authority to sell shall be irrevocable. In the absence of such an agreement ..., the authority of a factor to sell does not become irrevocable by the failure of the principal duly to repay advances.
"The" before "principal": is this because it is impilict from the words 'Goods are consigned to a factor' meaning that it is that principal who consigned the goods to the factor? Or is this implied from the context that the sentence refers to the principal of a particular factor (meaning that a factor is only a factor if it has his own principal, just the same as: here is a seller and the other guy whom he deals with is the buyer)? Or is there something different? What triggers the use of 'the' in the example?
'a' before "factor": my understanding is that when a thing is referred to for the second time, then it should follow 'the'. Why has 'a' been used there?
zero article before "advances": as in item 2, why has zero article been used there?
Many thanks in advance.