Resale of air guns is dependent upon consumer demand, the facets of which in the air gun world are constantly changing. A Bluebook for springers and pcp's would be extremely difficult to keep updated!
Considerations:
1) Consumer demand/lack of demand for a specific brand/model
2) Cosmetic condition, wood/metal
3) Air gun age, and buyer demand for that specific model
4) Caliber-fps/fpe...current, desirable, or obsolete
5) Mechanical condition...Poor to Excellent (...and by whose determination?)
6) Availability of rebuild/repair parts
7) Mfgr. still in business
8) Realistic market pricing of gun's worth
9) Single shot, semi, or full auto
10) Heavily modified or OEM
11) 2020 market influx of Eastern Europe and Asian air guns, some very high quality and high priced, some very high quality and low priced, some usable only as jack handles for changing tires!
Your best bet on sales price of a given gun is to watch the "Classified" ads and see what the day's market value is for a specific gun. Be cautious of posting a "WTB" or soliciting a specific gun to buy, the scammers are legion in number and recourse to get a refund is near non-existent.
A current "Bluebook" should get you in the ballpark, but remember it's someone's opinion based on available data. Buyers and Sellers have vastly different thoughts on a used gun's real-time dollar value.
If you are buying watch the Classifieds several times a day...good guns sell quick! If you are selling, find a similar gun for sale and price yours $50 cheaper, CONUS only, shipping fees extra, Paypal F&F, cashiers check, or domestic money order.
That's my advice based on experience. Your mileage may vary, and further input respectfully appreciated.
Kindly 'Ol Uncle Hoot