You should emphasize that free software gives the user control over their
computer, and that free software does not carry the same restrictions that
non-free software (you might want to call non-free software something like
"licensed software" to explain that the user is only licensed to use it, and
that they do not actually have ownership over their copy). Emphasize that
they are fully within their rights to download more software and share it
with friends, family, and neighbors, while noting that non-free software
doesn't allow you to.
Don't use the "Microsoft/Apple/etc are evil" line too much, but it might be
helpful to compare the GNU GPL to a standard Microsoft software license to
emphasize the difference. See this
http://asyd.net/docs/misc/comparing_the_gpl_to_eula.pdf which has a verbatim
distribution license.
Since you are selling it preinstalled on computers, hardware issues will be
moot. You can also, if need be, sell wifi cards, printers, and other
additional free-software-compatible hardware so that they get a true 100%
free experience.
You are not just selling Trisquel, you are selling the free software
movement.
