0 I think it comes down to what you intended to say/convey, mixing tenses, and understanding the difference between 'will' and 'would.' If you go strictly off of your premise that there's a person seeking to buy a pen from you and you'll have the final say, then you must use 'would'--'will' means it is a definite that's set to occur in the future (whereas 'would' is conditional). 0-