[Hectic]

Or just do like i did check ehobby and intruder (baiscally the cheapest and most expensive two places) get an idea of what the stuff costs new and make a reasonable discount based on the fact its used (and just how used it is) say between 10-40% off retail depending.
But of courze if you have somethin that is a limeted run or super rare or "classic" or no longer in production you can likely ask full retail if not more