I've had that question from customers. The only practical way to do it on a print on demand site is if you know the client wants it is you order it ..when it comes in you sign it and forward it onto them with whatever markup.or they order it..when they get it they forward it to you, you sign and number it etc and send it back. Some people do want them signed and numbered. The only thing is its not really limited as others can order it as long as it is available on the site. However you can have a limited number of those that you will number and sign. It would be nice if there was an option for that. Many artists will not participate on a print on a demand site but would participate in a limited print on demand site with signed and numberd prints.There is probably a sizable market there waiting to be tapped.