Capital One Financial, the nation’s third-largest issuer of store branded plastic, said Monday it agreed to sell its portfolio of Best Buy credit card accounts to Citigroup, just two years after buying it. The move took some bank analysts by surprise because Capital One had recently been bulking up its store card business. Officials at Capital One declined to provide details about the decision. The McLean-based financial firm also did not disclose the sales price, but said that the value of the Best Buy accounts is $7 billion. Capital One said there would be no significant gain or loss on the transaction, which it anticipates will close in the third quarter. Capital One’s stock fell about 1.7 percent in regular trading Monday. Just two years ago, Capitol One leap-frogged to the forefront of the store card business with the $2.6 billion purchase of HSBC’s U.S. credit card portfolio, which contained 23 retail partnerships including the Best Buy portfolio. “It caught us by surprise because a big part of Capital One’s story was buying [the HSBC] portfolio, and they’ve sold a pretty big piece of it,” said Sanjay Sakjrani, an analyst with Keefe Bruyette & Woods. “From what we’ve heard from Capital One, strategically it seems the two parties had a difference of opinion and felt it was best to terminate the contractual obligation.” Capitol One made its foray into the store-branded credit business in January 2011 by snagging the credit card portfolio of Canadian retail conglomerate Hudson’s Bay. That deal was followed up four months later with the acquisition of Kohl’s department stores’s card portfolio, which gave Capital One more than 20 million accounts and the right to issue cards to Kohl’s customers.