I have no experience in this so be skeptical of this. It might be a good idea to bring on an adviser for this sale? Perhaps talk with a VC just to help on the sale for some flat fee -- say 5k$ + 3% of the sale - transactions fees? Use your own numbers of course.

If they are offering serious money (being BigPharma I imagine it isn't peanuts) you will want this handled professionally. Peace of mind that you are getting a good deal and while they company maintains interest.

Kudos on the success!