Sorry, I wasn't clear-- you are correct, you don't have to register to pursue, but you do have to register a copyright to bring a lawsuit for infringement.
a good site is here-
http://www.copyright.gov/help/faq/faq-general.html
dcalvin- the probe is a factor because unless you have permission to copy a piece from the designer, it is technically copyright theft to do so.
Folks- we all do it... what I'm saying is that unless the works are ours alone, we have no legal rights against someone selling the items elsewhere...
Even more strangely, many of the patterns come from google sketchup. The user's agreement specifically says that the models there can be used royalty free. This means that technically I am within my rights to sell the models I make from sketchup files
http://sketchup.google.com/intl/en/3dwh/tos.html
section 11.1 c is the part about end users
I'm certainly not in any way casting stones here, I'm as at fault as anyone else.
Lawrence