I have successfully used transatlantic crew from Sailopo.com. The owner told me to get a list of available crew and pick my choices, then talk to him before finalizing the arrangements, as he has sailed offshore
with many of his potential crewmembers. His applicants pay him an annual fee to get the opportunities list, so they are motivated and usually financially sound. For the transatlantic sail, we offered airfaire to and from the boat, and got over 30 applicants. The two we used worked out fine, although one had to fight queasiness every time he smelled diesel exhaust
I also made it a point to talk on the phone
to each candidate--sometimes you can tell how tightly wired someone is in a 5 minute conversation.
A short shakedown sail, like from overnight from one Canary Island to another, would be very helpful in avoiding problems--sometimes people are fine until they get short of sleep or otherwise stressed.