You can't actually "reply" to an article in the way that you're describing. You'll want to submit the article, and part it in the category it corresponds to during submission. I do believe Chris informs the buyers when articles they requested have been written by writers.
I assume then that I add keywords in the title and lead, that the requestor had written, to make it easier for Chris, and the requestor, to find my article.
Many of the requests look interesting, so I'll do them.
Yes, you want to make sure you make your title clear that it is a match for the article. There is also a short summary where you can go into more detail. Finally, you put your article in the right category when you submit it (there are categories to select) since this helps Chris and the client find it.
Keep an eye on the recently purchased content since that will give you an idea what has sold already and what topics are hot.