I don't know where commercial AV technicians hang out more.
On job sites or at commercial AV contractors.
I am looking for a fair price, not top price. This way I can purchase different equipment to complete the AV system. Anyway, any advice is greatly appreciated.
"BNC5" is not an Extron model number and it sounds like you may be describing a five wire bundled coax. Extron offers several models of such cable. The M59-5 and RG6-5 are only available in 500' spools, so unless the 700'+ you noted means more than one spool that would suggest it is Extron MHR-5 cable, which is available in both plenum and non-plenum versions. A prospective purchaser is probably going to want to know the actual cable model and version.
One reason this is important is that Contractors and techs typically already have connectors, crimps dies, etc. for the specific cable types that they normally use. If they would have to purchase additional hardware or tools in order to terminate it, that could make the cable a rather expensive purchase.
If they do normally use that cable then Contractors often purchase cable and hardware in bulk quantities to obtain lower pricing. So what is a 'good' price to them might equate to your taking a significant loss and might require a 'something is better than nothing' view.
In my experience, Contractors and techs are generally wary of purchasing partial reels or boxes unless they know exactly how much they are purchasing. Nothing personal and I don't doubt that it is 700+ feet of cable, but how does someone confirm that? Is the the difference in cost for a new, 1000' reel worth the risk?
Because of all this, your best bet might be a non-professional who is looking for either that specific cable or something like it and that knows that 700' of cable would be plenty for their application.