The two leading makers of three-dimensional printers sold off with gusto Monday. 3D Systems (DDD) led the rock slide, losing 14% in five times its average trade. Peer Stratasys (SSYS) shed 10% on a big volume bump. Stratasys stopped at its 50-day moving average and 3D Systems stopped well above its 50-day line. The level of selling in both stocks was serious. In 3D's case, shares fell on the highest volume since a breakout in April. The main news