My research indicates that the estimated retail value of your signed Ty Cobb baseball would be $13,000-15,000 assuming it has been certified. Although you know it is genuine, a collector will want to have certification from an independent service as proof.
This is one such service that is nationally recognized.
Certification of Baseballs
In general, a private seller to a dealer, via consignment or at auction can expect 30-50% of estimated retail value.
Insurance replacement values are usually about 20% more than retail values.
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