Twitter is in negotiations to buy TweetDeck, the London-based Twitter aggregation service, for an estimated $40 million, sources said. Neither side has commented on the deal. TweetDeck, which employs only 15 staff members, was started in 2008 by 36-year-old computer programmer Iain Dodsworth and allows users to customize their Twitter feed. TweetDeck has received $3 million in funding, but has never turned a profit. If an agreement is reachedl, it will be the sixth acquisition for Twitter, only five years old itself and fending off rumors that Google or Yahoo are attempting to take over the microblogging site.