Hi Paul, while you cannot sell the software directly, I believe you
could legally sell a kit including PDA, GPS, etc, and include a CD
with the software along with setup and configuration instructions as
part of the kit. You just make it clear that you are not selling the
software itself, only providing it with the PDA as a convenience, and
that it can be downloaded from (site) for free.

As a software developer myself, I'm sure that contacting the
developers to ask about the licensing agreement would open up plenty
of opportunities to profit from the product indirectly.

A good example here is Linux, a free open source unix based operating
system which is currently included in plenty of commercial electronics
products and computers. Even the linux distributions themselves
(RedHat, Ubuntu, etc) are commercial products (the support and
packaging) assembled from a free GNU product, thus they are selling
their installation tool, packaging and support for free software.

Many people will choose to buy an entire package ready to go with
instructions over locating and assembling hopefully compatible devices
themselves and struggling through the learning curve. Interestingly
enough, my confusion over which devices and software are required to
run XCSOAR is how I ended up in this thread today in the first place.

Tom