If someone had a Touring they would have to run wires to make it work or if someone had a Grand Touring they would have parts to replace if they had problems with there Bose and they could get it much cheaper than buying at the dealership.
As a former MS-8 user I can say you'll likely be super frustrated working with it at first (I was), but when you get it setup correctly it will sound amazing. I'll be interested to know how this goes for you! Mazda likes to simplify things in their audio system design so the head unit should have 4x pre-amp level outputs to the BOSE amp which you can just route directly into the pre-amp inputs on the MS-8. Not sure if you're planning on running external amps or not, but as you might know if you plan on using the MS-8s internal amp for your speakers it's not all that powerful (20Wx8 @ 4 Ohms & 30Wx8 @ 2 Ohms). Might be worth thinking about keeping some of the BOSE speakers if you don't already have replacements for all of them just so you have matching power. The MS-8 will work its magic and make a difference regardless.
But to answer your question I'd think if your factory speakers are in proper working condition you shouldn't have any trouble selling them.
thanks, guys. The MS-8 sounds pretty complicated, but it might be worth it in the long run. The bose sounds fine for now, just curious about the future. I will probably just add a sub for now and see how that goes.