Can an AR-15 rifle fire without a sear block?

Question

I'm thinking about buying a NIB AR-15 rifle, but the seller has stated that there is no sear block.

What exactly does this device do and why would it be missing?

Answer 1

In a firearm, the sear is the part of the trigger mechanism which holds the hammer or striker back until the correct amount of pressure has been applied to the trigger; at which point the hammer or striker is released to discharge the weapon. The sear may be a separate part or can be a surface incorporated into the trigger.

The term 'sear' is sometimes (incorrectly) used to describe a complete trigger group.

i do believe it is still able to be fired with this removed.

Answer 2

It's a chunk of metal added to the design of the semi-auto AR-15 that blocks you from adding a drop in auto sear.  The spot that the auto sear would fit into to allow you to convert your weapon to full auto is blocked by the sear block.  

It was a knee jerk reaction by Colt to try to appease the "assault rifles are evil" freaks.  Most manufacturers of AR-15 receivers didn't follow along.  I believe that even Colt has dropped the sear block recently.

If you don't plan to ever break the law by illegally converting it, don't even worry about it.