The Bible says Jacob's brother sold his birthright for a mess of stew. Today, warn financial watchdogs, America's pensioners are in danger of accepting what can sometimes be a comparably smelly deal: "Sell us your pension benefits," say advertisement, "and we'll give you one big lump sum now, up front."For somebody who's facing doctor bills or has missed a mortgage payment, a lump sum can be tempting. Partly for that reason, pensioners in growing numbers are accepting such offers, say financial regulators. But not all offers are legal, and some contain hidden costs.