MS13 motion sensors work fine with RFXCOM. The MS13 operates at 433.92MHz and my RFXCOM has a 433.92MHz receiver. The only problem with the MS13 is its ridiculous price tag.
Enter the MS14A. It is the US equivalent of the MS13, it’s weather resistant and can be found very cheap on eBay. Big John sells them in packs and ships to Europe. (Only takes a few days).
There is one problem with the MS14A. It operates at 310MHz which is not supported by my RFXCOM. I could have ordered a second 310MHz receiver but in the Netherlands, the 310MHz frequency is not allowed. My other concern was the range and reliability which is far better on 433.92MHz.
Thanks to www.domotica.famschenk.eu I learned about converting the MS14A to 433.92MHz. After the conversion, it operates perfectly fine with my RFXCOM.