I was one of them. Keep in mind when someone comes to the forums and complains about lack of sales they still have the ebay experience in mind. They do not know anything else. No need to think outside the box when all you have to do there is list. Oh and service your customers.
A welcome letter would immediately inform them they will not succeed if they follow the same way of listing they have become accustomed to. If the new folks decide not to thoroughly read the welcome letter and take it's advice (knowing it's from eBid itself) then there is not a whole heck of a lot anybody can do about it but at least everyone knows they were given the information.
Posts such as "I listed 10 items and have not sold a thing for 2 weeks" is already answered in the welcome letter as well as here in the forums. Complaint posts can go on and on but pretty much all the same. Just boils down to where they came from and what they are used to. Pointing out eBid is unique and different along with informing them how to succeed might just squelch the complaints.