Hello everyone. Thanks for your answers and advices.
Now let's go with order:
@ Guido Vrola: thanks for the pointer, I come from the hospitality field and for me it's first I deliver my work then I get paid. I didn't think about that.
@ Marcus: Thanks, but I got no damage at all. I got in touch with Dalia and she told that maybe CGTrader could process payements in the future. I guess this could be considered if failed payements become a real issue.
@ Burak: sorry to hear that, I hope you didn't loose much.
@ Zoubeir: I break no military secret in saying I scored only one sale on CGTrader. It was worth two months of sales on another site where I score three per month. If I scored that sale there.. Never mind. Anyway, I'm part of a Facebook community of vendors dealing with that site and the general opinion about CGTrader is "very positive". Interesting market growing by the month. Do your thing and upload good models, you'll get a result.
@ Supercigale: thanks for the tips.
@ John Hoagland: no contract involved, only a foolish 30 dollars agreement. There are E-mails and private messages of course. Anyway, it was a somewhat rushed job. I think I could refine the model and get something out of it. For the moment it stands at the end of a trail of projects...
I don't know about the "fame and shame" method. If he shows up again he'll change nickname for sure. Since he has a GMail account I got a name and surname (does it work like this for all mail providers?), but I don't know if they can be made public at all. Anyway, not in this case for 30 bucks.
@ firdz3d: That could be an idea, but imho this line of solutions should be treated with caution. It tells customers "we definitely don't trust you". Again, if failed payements become a real issue on CGT that's a good idea though.
@ CARTOOMOTION: I really liked your answer, firm, to the point and very polite. I think you gave everybody another good pointer :)
Next time I feel I can handle a custom work I'll keep your advices in mind.
See you all!