There's nothing dodgy about it. It's called BMW Navigation system Business --- the exact same picture (with the wood trim - which I don't like) is shown in my Z4 2007 coupe catalogue as an option. It provides traffic directions by the voice computer and arrows on the display ... I don't have it, but that's basically most of what I would want from a satnav device.
The obvious disadvantage is the absence of visual information. It's also a bit ugly compared to the sleek and elegant normal BMW stereo.
But I think it has 2 clear advantages:
1. The full more expensive satnav has dated badly because the display is low-res and not interactive.
2. I think looking at satnav devices whilst driving can be distracting or even dangerous ... obv less so than trying to read a map ... but simple voice commands is probably much safer.
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Clarification: My comment above refers to this:
I don't know what is installed in that ebay advert.