They recently required all members to contribute money. It was more or less voluntary before this. Your contribution buys you a subscription for a year and so many classified adds. More money, more adds. They are randomly picking 2 (or more) letters in the alphabet each month and enforcing the new policy for anyone whose last name starts with those letters. If you have not contributed in the last year, you won't be able to login without contributing.
Astromart has a post about the new policy, but it can be hard to find. It tends to get buried in new posts.
Ya well ... This is a bunch of B#%| Sh@% ... Random names... Alphabetically is not quite random Herb! I can't believe this. I actually thought A-mart was a good site. It's not like I can't afford $15.. It's the principal of the thing.