Chuck's Provamo registry is a great resource, and cars for sale advertizing is free, but given that the 'for sale' section isn't open to everyone, nor does it show up in search engines well, I would use other means to supplement that.
As they always say "you get what you pay for" so free advertizing will not necessarily get you to the masses. In my opinion eBay is the cheapest form of advertizing there is, gets your care Worlwide exposure for a few bucks and something like $125 if it sells.
Hemmings is a good resource too, online and printed advertizing for less than $100.
That coupled with a comprehensive photo package and descriptors of history and mods etc. to fire off to anyoen who enquires.
Good luck,
Julian
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