Set analysis is evaluated once per chart, so it won't look at the dimension values (that's what you probably intend by using Date of sale >= Date of last meeting).
So I don't think set analysis is the way to go here.
You should be able to use a conditional embedded in the sum to check if the date of sales is after the date of last meeting:
=sum(if([Date of Sale] >= [Date of last meeting], [Value of Sale]))
Another approach would be to use an IntervalMatch between your two tables (creating a second field for the interval end in your customer table with date of today). Check the Intervalmatch Load prefix if you are interested in that.
Hope this helps,