Any reason not to make this trade?

I have an opportunity to do a straight trade: my 85 f1.8 for a 100 f2. Both lenses are mint.

I've had the 85 for a couple of years, and to be honest I've never really taken to it. I appreciate the results it produces, but the purple fringing is a pain in bright conditions, and I find the focal length to be neither one thing or the other. I use it mainly for portraits and indoor sports.

I also own a 60 macro, which I love and use much more than the 85. I think I'd enjoy the length of the 100 more; 85 is sometimes short for sports.

Is there any reason not to do the trade? As far as I can see, the lenses seem to sell for around the same price on the used market; if anything, the 100 tends to sell for a bit more.

Nope, seems fair enough to me.. technically you'd be getting the (slightly) better end of the deal, since the 100 f2 sells for a tiny bit more. If the 85 doesn't work for you, and you think 100mm will, go for it.