March is traditionally a strong month for auto sales, as people thaw out and hit dealerships. But maybe not this March. It could be just the sixth month since 2009 in which the U.S. auto industry posts a year-over-year sales decline, a sign that the big post-recession boom is slowing. Right now it could go either way as volumes are tracking flat with last year. TrueCar sees a 0.8% decline; Kelley Blue Book a 0.3% dip; and LMC Automotive goes the other way, with a 0.3% lift.