[sulam]

Typically these are signed upon employment, not after you've left employment. But the reason they are usually signed then is specifically to avoid the situation the seller is now in, so it's reasonable for them to want this.

The usual way to deal with side projects is to specifically list the things you own and want to protect that should not be included. This is often a very small set of identifiable things. If you're worried this might be an overly broad set, then you can go the other way and try and list all the work you did while there. They will then need someone to review it to make sure that's everything they need, and then the buyer will need to review it to make sure that's everything they want. If this is an asset transfer, they will also need to make another list which is the things they are specifically not buying. So you make things more complicated doing it that way, but it's certainly your right to do that since they don't have an agreement with you already. Their fault for not giving you something when you started.