The obvious way would be to open a (free) account on Marktplaats, message the seller by using the "Bericht" (message) button, and see what happens. Most people selling interesting framesets won't be too surprised, I'd think.

Another approach could be to get in touch with an experienced local trader in such wares, who might be willing to handle the pickup and shipping - or even the whole transaction - for you. One man who comes to mind is Frankie van den Elzen of Thevintagebicycle.com. Nice guy, knows his stuff and his prices are reasonable, IME.