Ideas for a price tag on parts from old city set? I took advice from my previous question and did a bit more research, but I have somethin specific.

I realised I have had a City set with a few more "unique" parts. The set is 7900 Heavy loader. I can't seem to find any information on pieces, but I know it contains a unique Trapezoid, modified tubes, smooth large grey plates and I think I can get something from it.

My question to you is more of a plea for advice. If you're not familiar set, you surely at least have a better eye for old set value than I do. Any suggestion for next move is welcome!
Bricklink price-guides:

The trapezoid:
https://www.bricklink.com/catalogPG.asp?P=55767&amp;ColorID=86

The smooth plates:
https://www.bricklink.com/catalogPG.asp?P=48288&amp;ColorID=85

Don't know what you mean with modified tube, but this part might be considered a tube and unique:
https://www.bricklink.com/catalogPG.asp?P=40244&amp;ColorID=85

Not exactly the most expensive pieces, so I think you'll make more if you sell the set as a whole rather than parting it out...

The Trapezoid price surprises me a bit as I think it is indeed somewhat unique, it appears in 3 sets but the other two sets it appears in, it appears in orange. 
I would have thought it's a wanted piece as it can be used to make bridges in train layouts...

The large tiles, I wouldn't pay even the rather low bricklink price for them as I have gotten hold of quite a quantity of them when last visiting Legoland Germany, where they were available in bulk at the "Fabrik" shop.