Many people - both collectors and dealers - will arrive by plane as well. Looking at the map of this part of Europe, one can see that Maastricht/Valkenburg is situated almost right in the middle of the triangle of major airports from Amsterdam - Bruxelles (Belgium) - Düsseldorf (Germany). Even Cologne/Bonn may be considered. As well as Eindhoven Airport (Netherlands) and Weeze Airport (Germany).
This is the most convenient destination. You don't really need a rental car, unless you have lots of goods to carry or have other plans too. By train you can arrive within 3 hours in Valkenburg (see "By train").
Is very convenient if you get yourself a rental car and drive in the direction of Maastricht. By train you go into the direction of Liege (Belgium) and change for Maastricht, then 10 minutes direction Heerlen for Valkenburg but that takes longer than by car.
Düsseldorf or Cologne/Bonn airports:
As far as I know these are only convenient if you get yourself a rental car and drive in the direction of Aachen… By train you also go direction of Aachen, then Heerlen and Maastricht for Valkenburg. But that may take much longer.
Some may use the Airport Eindhoven or Weeze Airport both as a very good alternative. First one is about 1-1,5 hour away from Valkenburg. The other one some 2 hours. Eindhoven is an easy bus-train connection. Rental car is advised for Weeze. The advantage of choosing these airports is the wide choice in many destinations all over Europe, usually from smaller airports too, because the connections are by low cost air companies. A quick glance on the arrivals/departures listing of Eindhoven Airport e.g. shows destinations such as: London, Bristol, Dublin, Milano and Roma, Marseille, Madrid and Barcelona, Stockholm, Budapest, Prague and Katowice maybe more too. Apart from several typical holiday destinations such as Budrum and Tenerifa.
The general page of Weeze Airport lists destinations all over Europe not all from and to Eindhoven or Weeze however. You may have to book for one of the other airports used by such companies as Ryan Air. Charleroi (Belgium) e.g. near Maastricht too, but no convenient public transport from there to Valkenburg. Also Liege (Belgium) too, has an airport. Mostly for holiday charters. Maybe something to look into also.
Also I got the impression it is mostly Ryan Air offering these connections... Wizz Air has announced more destination in Eastern Europe too. Check their web site. Also Hamburg International has connections with other European airports, mainly holiday destinations. But also Berlin.
Have a nice flight!