I haven't the foggiest notion of what is being asked of me here. To elaborate on my question, can you market a collection of essays (some as yet not written) by submitting say five or so, three of which have been published?
Why would a publisher want to republish your previously published essays? If you are Barack Obama, this will work, or if your blog has a million readers each day. Otherwise it is quite unlikely.
Unless you are famous or your work has acquired a cult following, publishers want original content. And don't try to trick them by deleting the essays from your blog, because if anyone remembers reading the contents of your book elsewhere before and the publisher learns of this, you'll have a huge legal problem.
Agents or publishers commission a book only if the author has proven to sell. Newcomers -- unless famous like the Pope -- always need to submit the beginning of a finished work. How much of the beginning depends on the agent and will be explained in the submission guidelines on their website.
Books collecting small forms -- a collection of poems, short stories, or essays -- invariably sell fewer copies than novels or monographs. For these, all the above is even more important. Publishers usually only publish collections by authors that are well known. In that case, successful prepublication -- in a print magazine -- is not uncommon. Web publications are always a problem.
I haven't the foggiest notion of what is being asked of me here.
i'll assume that you mean "in order to get an agent"
This isn’t Hollywood where you need to have a reel, or an art school, or gallery, where you need to present a portfolio, or a university where you need to present sample essays.
You need to already have sealable product before you approach an agent. He won’t take you in and groom you on the strenght of some sample writings…
(Most likely he won’t groom you to become a better writer even if you have great manuscripts and potential "best-seller" stamped in ink on your forehead)
Like @what said, you need a polished finished product before you approach an agent. It is extremely unlikely that the agent will take you on based on some un-commercial essays, or essay collection.
Either develop these essays into a novel or chuck them away and start to develop a novel lenght book.
also do some basic searches like reading Q/A like: How can I make a collection of essays / arguments more attractive to publishers?