While stationery cut outs are collectable in the US, they have zero market here.
Note that stationery cut-outs could be glued to envelopes and used as stamps at one time. A stationery cut out attached to an envelope with the postmark tieing it to the envelope showing that definite use would be of interest to a postal historian here.
Just to clarify cut-outs and collect-ability in the USA.
Cut-outs from envelopes are collected, usually worth a discount off the value of a complete envelope. Cut-outs from postal cards and aerograms/lettergrams/lettersheets are not regarded as collectable and are not valued in Scott. Those must be collected in their entirety, or are considered to be not much more than fillers.
In the USA, the use of a postal stationary cut-out for regular postage (gluing it on an envelope or post card) is not allowed. Some do get through, though, and fit in the "unauthorized use" topical that some people collect.