How would you design these postcards?

[Claude Whitacre]

I don't think the colors or font are important. Black and white is what I'd use. It's the offer.

I'd say "Mikesbikes.com just became available! Call 1-46-567=-3456 to see if it's still available. This letter is going out to 5 Mike's Bikes shops across the country. First call wins!"

Maybe on the other side say "When customers go to Mikesbikes.com.....will they find you or a competitor?" 

Everyone wants what they can't have. Everyone wants what they think someone else wants. You'll probably get at least one call. If not, I'd call them, assuming the price of the URL is enough to justify the hassle.