I don't know who your target advertisor or target advertisee is
But, if I were an advertiser, I'd worry that people willing to spend time looking at ad messages for money don't have a lot of disposable income... and thus they are exactly the type of people I don't care about advertising to.
You are right that "pay to send" solves the spam problem. But gmail filters spam so well that this is already a solved problem.
Additionally, the sidebar ads in google or facebook are auctioned off to the highest bidder. So users are already receiving improved targeting based on pay-to-send. The only problem I see this solving for users, is that they collect the money instead of facebook or google.
That is very little money on a per-user basis. So it's not solving a meaningful problem.
I agree that there is huge potential in "pay-to-send." But this marketing appears to be off-the-mark.