It's not just the UC announcement that caused the market to quiet down. The mobilization of the site has really messed with a lot of people who liked the original site and can't adjust to the somewhat infuriating changes that have gone on, rendering it unplayable for some with the new hoops you have to jump through to get around.
That being said, UC sales have dropped significantly while people wait to see what the UC change on site will be. Maybe you'll be able to make any pet you want UC and so your favorite names that you already own can be UC. Those with names they love will wait instead of trying to pick through the scraps of names that are left at this point in time. Names have always been a struggle. There are plenty of people still buying UCs though because they don't want to wait or they don't care and they still want a tradeable original version.
What the announcement did do was decrease the value of UCs and increase the value of Real Word/Real name pets since now those are the special rarities. It's an interesting turn as RWs had died off a bit before and lost a lot of value, and now the scales have tipped the other way.