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Someone told me that Jessi was originally from Turbosquid - I'd never heard that before, but if that is the case, couldn't someone else go buy that same base mesh as well? And wouldn't that be legal? At least the head job was sufficiently better made in the new one...

I dunno who told you that, but P6 Jessi was put up for sale there shortly AFTER Poser 6 came out. I reported it to TPTB at E Frontier at that time and it was soon taken down from TurboSquid. Searching TurboSquid just now I was able to find head meshes that are based on P6 James and look to be simple faceroom morphs that have been exported out and put up for sale there. I also found the Poser 3/4 Casual Woman there too! Just goes to show that TurboSquid still doesn't do any copyright checking of the products people upload there.