Best Buy said Friday that it has authorized its store employees to match online prices of competitors in some cases, as it ramps up for the holiday shopping season. Best Buy and others already offer price-matching guarantees for local competitors’ brick-and-mortar stores. But stores don’t usually match online prices, since those tend to be lower. Competition between online merchants and discount stores is expected to be fierce during the holiday period — when retailers can make up to 40 percent of annual sales. A Best Buy spokeswoman said the match applies to appliances and electronics and will be given on an ‘‘if asked’’ basis.