Just to be clear, you are looking to sell sponsored posts, not buy them?
Before I start, there are a few exceptions to this but it would need to be very specific, or niche offering, where I could actually see, if it didnt have history already then it would actually become something, and I'd benefit greatly from getting in early at a cheaper than chips price.
BUT, in the general sense and for 99.9% of all of these emails, I get around 10 a day offering such and then another half a dozen who follow up to a non-replied email, all go straight into the bin, anyone actively selling in that way would be of little to no value as its either being done on scale with little else other than sponsored posts, or the site isnt offering enough value in its current state, if neither of those were true there would be no need for the cold email approach.
And if someone cold called me, taking my time for something like that, they'd last at least 3 seconds. I can't think of anything more annoying.
That said, if it didnt work then no one would be doing it so there must be some muppets willing to take up the offers lol