Average value on conditions

[AGA]

Hi,
I am looking for formula, which will sum values on conditions.
Namely, as in example I'm looking to estimate column 'sum by year'. (the values in example are estimated in 'primitive way', which will take too much time for huge file).
I need to find an average value of transactions by year, where number of transactions varies. Moreover I need to have value of 0 if the raw does not correspond to the last transaction of the year.
I'd appreciate your help.
Thanks,
AGA