I'm selling a BCM middy upper. Potential buyer planning on putting it on a 1999 vintage Colt Match Target Comp lower. Any problems with the front pivot pin matching?

Maybe the best thing to do is ask the potential buyer if he could actually measure the lower pivot hole, if it is small .250" (1/4") then it's good to go.

If he have a large hole approx. 5/16" (.3125 - .316") you could ask him if he's willing to use a pivot pin adapter, take a look at Brownells and other AR parts dealers.

If I remember correctly Colt was changing the pivot hole size from large to small, several years in the early '90s, by the late '90s they should all be small pivot holes (YMMV).

This thread has some good info.  Look  towards the bottom and there is a post with the description of the various pins, sizes, time lines and pictures.  It may help you out.

http://www.ar15.com/forums/topic.html?b=3&f=123&t=452969