When it comes to the marketplace, you take the good with the bad. I've had successful interactions with several sellers, while being ignored by others. As long as you don't let the non-responses bother you, using the marketplace is still a win (IMO).
Also, if you avoid messaging sellers who are no longer active here (e.g. "Warning: This User has not logged into the site in 2529 days"), you'll be ignored less often