Penguinflies pretty much nailed it for you. It all depends on who is in the market for such equipment and what they're willing to pay. The DC-9s are probably just about fully depreciated, so they're worth the value of the scrap aluminum they're made of, especially if it's due for any sort of major inspections, maintenance, or overhaul, C check, or D check. The CRJs are probably worth significantly more. You can get a list of certified air carriers from the FAA; perhaps you could get in touch with them and see if they're looking for any used equipment. Otherwise, you could put an ad in trade-a-plane. So do you have four spare jets just sitting around?
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