Boundaries and being true to oneself

Thanks to SCS I’ve become much more confident and serene in my interaction with other people. There are only rare occasions when I need to set a boundary, as I am usually communicating my wills and won’ts, likes and dislikes beforehand. However, sometimes I am left with the nagging feeling of being too non-confrontational. A current example:

A guy contacts me with the intention of buying my website, which he has learned I am planning on closing down. During our meeting and subsequent email exchange it turns out he wants it for free. I say no, inviting him to make an offer if he wants. He follows up with an email listing all the reasons why supposedly the site isn’t worth any money and he is actually doing me a favor continuing it. I don’t react.

Case closed, but: All the while I feel like wanting to point out to him that
– it was him contacting me with a desire to buy, not me wanting to sell
– under false pretenses he has asked me to to supply website metrics which have taken me time and given him confidential information
– I find his verbal devaluation of the site’s worth quite insulting

Is this useful (being true to myself and, without expecting the other person to change their point of view, letting them know mine) or not? Thanks for your take on this!