Eventually, when there are too many sellers, even excluding non-legit sellers, the wards will still crash.
Well, the way I read their note is that not only are they increasing ward capacity from say, 500 to 1,000, but they're going to make sure that 1,000 retainers won't crash the wards.
Alternately, if they have a max level of retainers, and will just cycle the oldest retainers (even after removing the max price guys), they will be removing legit sellers from the market, also not really a great option.
I really doubt they'll simply go by the date they were inserted, and my hopes is that they don't automatically eject a retainer because the cap was reached.
However, even in a perfect situation scenario, a hard limit, no matter how high, as to have some kind of cleanup method; without that, you could find yourself in a situation where you have all 1,000 slots of every ward taken by players who have quit 3 months ago, hence why they'd mention "But we're going to have a system to lower the amount of retainers". They haven't given details. It could be something simple like activity (Did you sell anything in X days), inventory (do you still have things for sale?), or player activity (did the owner of the retainer log in in X days). There are more complicated ones, like "How often did your goods come up in the search results?", which would eventually weed out people who price so high that they don't even show up, etc.
FFXI's auction house limited it to 7 items for sale per character, and extra character meant extra content IDs and fees. That essentially lowers the strain per server (limited amount of items on sale at a time), but also means that players with tons of characters and more strain would also contribute more to the server upkeep -- a pretty good deal for Square if you ask me. Ultima Online
designers actually had lots of issues with the fact they didn't enforce enough limits on the game during its first months/years, and they had to keep upgrading their servers because they found some players were keeping ten of thousands of shirts and stuff in their bank. There's a pretty good read about it over there: http://www.mine-control.com/zack/uoecon/uoecon.html
Anyway, I never really cared or hated the wards, and I'm interested to see how they plan to increase their stability and solve longer-term issues. I'm a bit confused why they don't simply instance new wards when some of them start reaching capacity limit...