I believe "knocked down" is the technical term used when auctioneers knock their gavel to signify that bidding is over.I'm confused by the meaning of the sentence, regardless of whether you use "at" or "for".
"The vase was knocked down for £5000" means to me that someone was paid £5000 to knock the vase to the ground! (Similar to "His wife was murdered for £10,000"!)
"The vase was knocked down, at £5000" (note the comma) would mean that the vase was on sale at a reduced price, now £5,000.
If the vase had previously been on sale at a higher price you can say "The vase was knocked down to £5,000".
In order to use "for" I would have to change the sentence to "The vase was on sale for £5,000, a knock-down price".
I believe "knocked down" is the technical term used when auctioneers knock their gavel to signify that bidding is over.
In this case, I think 'at' and 'for' both work, but 'at' is part of the technical term - to knock something down at a certain price.
knock-down: Definition from Answers.com