I have a customer that has multiple buildings, that he might want to cover. ( I have a walk-thru 6-14) I know he has fiber running out to these buildings, but i think that it is all used up. If his copper feeders are not all used up, can I use these to feed my video 1,000 ft?

I assume you are talking about the Telco multi-pair lines???

You can use them, but be aware that you may encounter problems with ground loops and noise. Telco wire is usually rated as Cat-3 but on most of the existing wire I've dealt with there is little if any twist to the pairs. The less twists, the more noise is picked up.

Also, 1000ft. is a bit long for passive-passive so you may want to use either active receivers or active transmitters.

Look for some active baluns rated for Cat 3. maybe if they have data this might be the place for an IP camera