Celestron in particular seem to have it sown up in terms of dealers and territories. We have a Celestron dealer in Ireland and couldn't buy from the UK. At one time there were noticable differences in prices (now rectified though). I cringe at the low prices for their gear in the US though.
I have bought a few accessories from OPT without any problems, including a DMK camera. I haven't been caught for customs either (using the US postal service
) so I have had some great deals. Its always worth asking them what they are prepared to ship. Even with duties there must be big savings to be made. They are a great company to deal with.