Good luck with your sale if you do decide to do it.
Beautiful car and will never be worth to anyone as much as it is to you. However, low mileage or not, it is getting up in age and things will start breaking down, it's inevitable.
Enthusiasts (my opinion at least), do not buy 9 year old 5 series, and trade in offers will just anger you, there's no avoiding that.
I didn't need to, didn't plan to (or wanted to) sell either, but deep inside I knew it was time to let mine go as various bills started coming at me fast. It had more to do with "ugh, again" than money, sure I could afford it just fine, but have started wondering "OK, that's done, now what's next" - 9+ year old luxury cruiser will only end up costing nice chunk of money going forward. It depends what you are willing to tolerate and if there's something out there you like more.
Depreciating asset either way, this or new one - will cost money either way.