Thats a difficult one for me, as you know I don’t like using sphagnum moss as it remains too wet for me during the winter months but a lot of growers have tremendous success using sphagnum, So depending on the individual plant for example is it epiphyte or terrestrial will depend on your media choice. Masdevallias can be grown successfully in a variety of medium so long as its fast draining & moisture retentive which is essential for a healthy root system. I use small bark & a perlite mix which I find meets my requirements and have reasonable success. I would ask the seller what they recommend as they may well have just used sphagnum moss for a recent division to encourage root growth prior to selling.