Do you want cash and/or stock for the sale? Or, do you want a %age of future royalties of the sale (that might tricky if they want to incorporate it into a bigger product). (Or a combination of both cash/stock and %age)
Do you want to remain involved in the project, or even moved into the team that would continue to work on this? If so you should get that contractually agreed as part of the sale.
How much do you feel the project is work, and separately, how much has the company indicated they are willing to spend? If it is more than, say, $20k I would go and see a lawyer to draw up the contractual issues. Less then $20k, you can probably do this on your own given the amount is small. You could also incentivize the lawyer to negotiate on your behalf in return for a %age of the sale - particularly worthwhile if you are not a good negotiator. It's also a 3rd party, so none of the mind games that could be used against you by negotiating with your own boss/boss's boss/etc comes into play.
You might be wise to incorporate the project into a company, transfer the IP to the company and then sell them the company.
You should also review your code to make sure you are not using any OS code that has an incompatible license with selling the code.
Perhaps you could provide more answers and people could advise you better.