"Few could have taken advantage of this one-day 'sale' price." What does 'could have' mean in this sentence?
I would interpret this as meaning that not many people would have had the opportunity or means (been able to) take advantage of the sale.
I have no idea what the context of the sentence is, but the fact that sale is in quotation marks, in combination with the earlier part of the sentence, suggests to me (although this is mostly speculation) that it was a sale that few people would have been able to afford. For instance, the price of a $50 million dollar mansion being reduced for one day only to $30 million dollars.
The use of the quotation marks strikes me as sarcastic (scare quotes), indicating that most people with a standard amount of money wouldn't have considered even the sale price to be anywhere within their normal means.