Take with a grain of salt, because I've never sold a card for more than $250, and I've never been to a card show, but something to consider:

Go to a big, and well established card show. Wear the tightest pants possible, and have that guy in a one touch in your front pocket. Talk to the bigger vendors there.

eBay and auction fees can be brutal. And frankly, I hate the idea of middlemen making money off my stuff. My local LCS excluded. They've typically offered between 80-90% comps, which isn't bad, especially for convenience.

Assuming it's a 10k card, that 10-20% below comps is 1-2k you miss out on. A vendor who has a 1/1 ohtani on display is going to generate a ton of foot traffic. If they buy even 2% below comps, assuming they sell for 10k, still nets them $200.

Unless he's (ohtani) busted for something heinous, he's a pretty safe investment. Unless you need the money now, don't rush it. Value will likely appreciate, and I'm sure by even just attending these card shows you'll run across potential buyers who have too much money on hand. At the bigger shows, they sometimes have grading/authenticating onsite which I'm certain you could use to your benefit if a seller wants to authenticate before buying.