l have an innovation which is covered by a provisional patent which I'd like to sell
First of all, what you have is a provisional patent **application**. For now, the provisional patent application is not publicly available. This can be a big advantage to you. You can use this time to improve your invention and file additional provisional applications. If you don't plan on making/practicing the invention yourself, you should make sure to try to think of everything possible regarding how that invention can be improved. This would (hopefully) come naturally to a practicing inventor.
When you file your non-provisional application, your disclosure will be made public ("published") 18 months from the filing of the non-provisional.
While your applications are not publicly available, I suggest waiting until you file your non-provisional application(s) to talk to others. Quite often, provisional applications do not have the detail or the scope that the non-provisional will have -- due to the time spent on getting drawings accurate and making sure everything is included in your non-provisional applications. It is best to have a complete application before talking to practicing companies. Sophisticated companies will take advantage of information not disclosed in your provisional application to file their own patent applications.
If your applications are not publicly available (or even they are published), you should sign confidentiality agreements (non-disclosure agreements) with anyone you talk to about licensing or buying your invention. Keeping track of who you talked to with confidentiality agreements can be the basis for "notice" of the invention by an infringer -- when they eventually steal your invention.
You can sell your idea anytime, but a provisional application is worth a lot less than a non-provisional application which is worth less than an allowed application or issued patent. Provisional applications are generally not as complete as they could be -- and require additional time and finesse to complete the non-provisional application. In terms of appearances, if you are not willing to put in the money to file the non-provisional application, the invention is likely not worth that much money.
Companies as well as non-practicing entities may look at provisional applications, but not for long, and will not give you the kind of money that you may get from selling a fully examined patent. Of course, it depends on how good your invention is.
