Not a big deal, I don't really see what harm anyone can do with an IMEI though.
There, unfortunately, isn't another way to factory unlock an iPhone without the IMEI.
Many people on here know were I'm employed and have much of my info for previous trades (and I would be willing to share those details with trustworthy people) if that helps build trust.
From what I understand (and this is just from some quick random internet reading), someone could emulate or rewrite a phone's IMEI to gain access to the same plan at the original phone is on. So, theoretically, you could take my IMEI and put all sorts of fun stuff (text messaging, data roaming, purchases, etc) on my next phone bill. I am no expert, so it could be that this is completely wrong.
I'm not saying you would, as I really don't think you would, but I don't like to take chances like this anymore. If you could point me to a reliable source stating that this isn't possible, then I might reconsider. Also, the trust-building things you mentioned wouldn't hurt.
I don't have an official source but it's likely hard, if not impossible, to "create" a new/different IMEI number on an iPhone's Logic Board.
I understand your precautions and don't blame you for having them. I'll PM you for some more details about me.