Is the Tesoro lifetime warranty transferable?

No, original owner only. Also, making any repairs/changes inside the control box will void the warranty.

Can't blame them (Tesoro) for either really, a lifetime warranty is all but unheard of for metal detectors.

I bought a used Vaq and Cibola about 10 years ado, haven't had the 1st issue with either, don't know how old they were before I bought them.

However, the $50 charge is not unreasonable compared to what some other companies charge for non-warranty work.

Tesoro's warranty is really not a warranty on the machine, but a warranty on the relationship between the buyer and the machine. That might not seem to make sense, but consider this. If I buy a new Tesoro, and 1 week later I sell it, that machine has no warranty. But it's only a week old.