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I don't quite understand the issue of limitations very well, especially when it's oneself who decides them.
The question says:
it doesn't make sense to sort by most recent first...
we wouldn't want to list past webinars first ...
we would be burying upcoming webinars down the page that we'd want to prioritize showing since they're upcoming
Perfect, don't do any of those three things!
A filter has two priority functions, one for the user a and one for the site owner b:
a: help find a product easily
b: facilitate to show/sale the products
As the site owner, which one is your priority?
If the answer is a as a user I would be interested in searching webinars by (this order is my preference as a consumer, maybe not the generic one):
Price
Subject
Duration
Top sales
Upcoming
Offers
Specifics (author, ...)
Other
If the answer is b as a seller I would be interested in showing webinars by:
Top sales
Featured (weekly, monthly, by author...)
Most recent
Upcoming
Offers (buy 3 pay 2,...)
Others
If I want to see all the webinars, as a user I have no preference, I want to see them all, I can spend an entire afternoon playing with the scroll bar. But, as a seller, I have preferences, I would show an order prioritizing the best placed within a ranking of the b list.
BTW: this answer is generic, I don't know what webinars are about as it's not mentioned in the question