Ok, I thought about this, the answer is out there and it's more simple than I first thought.
Why do people make transactions?
(A) To buy goods
(B) To pay for services
(C) To transfer bitcoin to a different wallet
What percentage of transactions in a day are conducted by A, B, or C?
A and B apply to the OP's question, so if you add your assumption for A and B you'll arrive at a % of all daily transactions that can be attributed to payment for goods and services. Then multiply that percentage by the daily transaction volume to get your answer on any given day.
Let's say A + B = 85%.
On March 7th the Bitcoin transaction volume totaled $102M. (
https://blockchain.info/charts/trade-volume?timespan=all)
85% * $102M = $86.7M - this is my estimate of the amount of transactions used to buy goods or services conducted in a single day (March 7th).
What do you all think of this approach?