Let me be clear (who said that!).
I don't mean NO deposit - that's the only practical way to estimate production volume. The "I think I'd definitely maybe take one, if my Mom lets me" (responses to aluminum head for the slant 6) are completely worthless both as to intent and as a binder.

1. must have a non-refundable deposit, but with an escape clause like if no block in 1 year or price jump above $200 = 75% refund; after 2 years 100% refund; 50% deposit locks in the original price, etc. Make your own variables.
2. must be small enough to insure confidence in the seller ("you're not paying your rent with this, are you?").
3. must be large enough to insure confidence that the buyer won't walk away, like $500. That's just my guess - I've had people surprise me by walking away from $100. deposits