You know what, honestly I've been looking for 3+ months and I have never seen any stonewashed M390 no-choil XM's, so to me yours is kind of rare in that sense. I very much wanted one of those M390/no choil but I could never go through with it because I just don't like Working Finish...not enough to spend $425.
I don't know where everyone else is looking, maybe they have better sites or bigger group of sites to scan/Hinderer groups...but for me, the only one I've ever seen in that M390/no-choil configuration is in working finish.
That being said, I posted above that I don't think anodizing yours would do anything to diminish it at all, as long as it's done right by a reputable dealer and with the electro process.
Like I said earlier - I think, if anything, it could add to the price depending on the buyer. Me, for example, I like the anodized stonewashed XM's so that wouldn't deter me from purchase, it would be a draw. The guys here that know me know that I have had my last 3 XM-18's custom anodized by Whitty.
I prefer stonewash over WF and I like to get ano over non-ano. Not to say I don't like plain stonewash, I have one with no ano, too. So, just depends on the preferences of the buyers. Market is weird these days so there's no telling for sure which way it'll go but to me, you certainly won't lose money just because you anodize it.