I also don't like to split up pieces, and normally will not if there is a decent chance that they were originally a matched pair. However, in most cases selling them separately will result in a fairly decent price increase. I agree that eBay is the best place for maximizing potential profit as it provides a much broader potential buyer base than any other site that I know of.
Right now prices are depressed on most edged weapons unless they are somewhat uncommon and in Very Good or better condition. A dated M3 and M6 scabbard certainly falls into the uncommon category and if they are in nice condition you should realize a decent price.
Gary Cunningham - Bayonetman