Cassini is the tool used for the "Best Match" search on Ebay. Loads of rumour about it, but generally thought that it favours lower price, ending soonest, better pictures, sellers with previous high sales, sellers with better DSRs, few other things.
As a buyer you can use Cassini to your advantage; sort by Best match, select auctions and go to the last few pages (of many dozens or hundreds). This is where to hunt for best bargains.
As a seller, Cassini can shaft us, though. Very hard on low volume (private) sellers.
I think last year we had so much snow nr Xmas that Buyers have learnt to stop buying online around 10 Dec. Which is ironic because DH & I've both had stuff delivered RM 2nd class < 36 hours after placing order in the last week (not bought on Ebay, mind).
Xmas day & Boxing day plenty of people will have cash gifts they want to spend, so I am going to try one day listing on Xmas eve and then 3 day re-listings thru the rest of the FL-period (until 4 January, I think).