First - a caveat - I'm not a lawer. Not at all. But I care alot about intellectual property and not getting sued, so I try to learn about it a bit.
In no particular order:
- Double check your employment rules - when you took the job currently paying you money, what is your arrangment? Did you have to sign any statements giving your company control over all the code you produced? Even if it was a personal, unpaid project - corporate ownership can get you if you signed a strict intellectual property agreement.
- Used open source? - there are a few main open source licenses, read through and check them to see the terms for sale of a product with dependancies on open source.
- What deliverables does buyer expect? - Built code? source code? Also - what can you do to protect your code (obfuscation).
- Do they expect support? - be careful, in my experience with corporate customers, a helpful, free of charge "sure, just call me if you have a quick question" can quickly become time consuming. If you are willing to throw in a free couple hours, be very clear that you will give up to X hours of support for free. And be clear about what your billing rate is after. If you really don't want to support it, make the cost of your time very high.
- What sort of support do they want? - answers & configuration help? Bug fixes?
- What sort of installation instructions are expected?
- What do they own when they buy this? - a single installation for a single server? a site-wide license to install it wherever they wish? or --- worst case -- do they own this lock, stock and barrel such that you may no longer develop it and continue to use it yourself?
Get these answers cleared up, in writing, with signatures.
It's a good idea to have someone external read it to check for ambiguity.
It's an even better idea to draw up the agreement and have a lawyer read it - your lawyer, not the buyer's lawyer.
Avoid any nod/wink/handshake deals. Personal trust is great, but people change if the situation becomes stressful. Or people come and go within companies - the buyer today may be a different person tomorrow.