A shiny mark is just when the surface texture of the paint has been affected, a rub is when it's a bit deeper and some or all of the colour of the paint has gone, so you can see through to the base coat or bare plastic underneath. Shiny marks can be hidden with careful lighting so those models are often sold as 'photo show quality', because they're good enough to take a decent picture of, but the damage would be visible with the closer inspection your'd get at live shows. 'Live show quality' is the term for models without paint marks, rubs, or flaws which would spoil their chances in the show ring. Models with minor marks can still be taken to shows, of course, but they'd just be placed lower than their mint condition rivals - as long as the judge was checking carefully!
There's no such thing as too many models, only not enough shelves.