If you bought a registered lower, then the lower is the machinegun, not the sear. The sear is just a part like any other part. If you lose your sear, go to Brownell's and buy another one, they sell them for like $15.
http://www.brownells.com/rifle-parts...prod41732.aspx
Back Ordered for now. But I'm sure they're not hard to come by otherwise.
Either way the auto-sear in itself is not illegal. Now if you do not have a registered lower that that sear could arguably be used as a legitimate part for but you have some semi-only lowers. Then you could possibly, maybe, theoretically get snagged up on some constructive possession charges.