Advise for the seller
You need an export certificate from the FAA.
which should include the deregistration.
Also a paper from the FAA stating no claim is against that glider.

Advise for the buyer
First you must make sure the glider has already been licensed
in Canada, experimental class is not possible, unless you want to
go into the owner maintained class. You would have to make
a case for this as not every glider gets it. All regular gliders
have a regular airworthy certificate.
Request the history of the glider from the FAA (you get a CD
with every thing on it). Make sure the glider documents are up
to date, manuals and all.
You need the sales documents and that you paid the taxes when
you entered Canada with the glider. Keep the trailer separate and
bring it in as a homebuilt trailer, if you can.

I highly recommend you get in touch with a DOT designee first
before you hand over the check. It will save a lot hassle.

Unfortunately I did bring my glider in first and it took 4 month to get the
glider flying. It should only take 4-6 weeks. If all the paper work is ok
and all the AD's are complied with and documented.
Documentation is the key word. Everything has to have a paper trail.
Including radio and instruments. I was overwhelmed by the process.
The DoT designate took charge and took me through it.
$1000.00 were well spent. For that he even set up new, up to date manuals.
I also had help from the Canadian Schleicher dealer, Ulli Werneburg

Regards
Udo