It costs $50 US for his service/certificate ($75 if you want it anytime soon) ... and then you'll need to pay for the cost of shipping your coin to California and then back again => it's a bit of a gamble ... if it's a fake, then you're screwed ... but if he gives it the thumbs-up then at least you have confidence in your coin, plus you have a sweet David Sear certificate (which might be worth a few bucks on top of the price of the coin?) ... sadly, there aren't too many things that are free in this world
As stevex6 mentioned, it's a costly gamble to send the coin to David Sear. Might I suggest you personally return the coin, unannounced, to the dealer and explain your (and other's) doubts about its authenticity and politely request a refund. Who knows what he might do.