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The value is driven by the present condition, expressed as the remaining amount of factory-original finish on the wood & metal parts, as compared to new, and the absence of any modifications after it left the factory
Your Model 87 Deerslayer w/rifled bbl is in the latest BBGV (Blue Book of Gun Value)
as:
$185 in 60% condition
$200 in 70% condition
$215 in 80% condition
$235 in 90% condition
$280 in 95% condition
$320 in 98% condition
$385 in 100% condition (with box/papers)
The listed "values", however, are not the true values - which is how much somebody will pay for it today/recently.
IME, the best way to obtain the true value would be to place it for sale where the maximum number of interested buyers will see it (i.e.., not in a local gun shop).
As for selling to a dealer, keep in mind that they are not charities, and will typically offer half it's book value - if they perceive they have a market for it.
The best venue for getting the best return on investment IMO would be to list it in one of the online guns-only auction websites.
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