PowerSellers Unite tracks listings volume across a bunch of different sites. You can visit them to see the current volume versus the same time on the previous day. Normally that page is embedded in the site, but is currently a standalone page due to the high volume.
As of right now, it’s showing a decrease of 3.03 percent. Is that a big change going from Sunday to Monday? I don’t honestly know. The charts page isn’t currently working (just takes you back to the main page). I’ll check this every day during the boycott and update as appropriate.
A couple of days ago I wrote about the boycott. If you’re interested, you should definitely check out AtTheBat’s comment on it. I think he nailed the mentality behind some eBay sellers (not all, certainly, but some). I’ve got some more thoughts on this, and a reply to ATB that I’ll hopefully get typed up in the next day or two.
Also, I’d like to pass along some advice I’ve seen posted in multiple forums about the boycott. If you are participating and are a regular eBay seller, take the opportunity to explore your other options. Every seller can benefit from diversifying their sales channels.