In all honesty, this is so far removed from what a Marcus Miller looks like I would suggest this is a case of caveat emptor. Ok, it may have been passed off as one but it bears no similarity at all. The price (as you concede) is a give-away too.
If it does the job then that's good. Maybe it's a genuine neck. If that's so then you've got the basis of a good deal anyway.
How do you avoid being stung? Do some homework I suppose. Googling 'jazz bass marcus miller' immediately brought up pictures which show the control layout is completely different to your purchase.
That said, I think it would be interesting to know which well known site has even offered this as genuine as they should have not done so.