By Anirban Sen (Reuters) -Swedish payments firm Klarna Bank AB is nearing a deal to raise fresh capital at a valuation of about $6 billion from existing investors led by Sequoia Capital, according to a person familiar with the matter. The financing round is yet to be finalized, but if successfully completed would represent a stunning decline in the valuation of a once high-flying, buy-now-pay-later (BNPL) venture that at one point was Europe’s most valuable startup. In an emailed statement Klarna said, “As always, we do not comment on fundraising nor valuation speculation.” Last year, Klarna r…