Andsorry - I was not trying to turn this into a debate about marking gift vs not marking gift on customs forms.
I would just send an email - something to the effect of -
Dear Buyer,
Thank you so much for your purchase! I received your request to mark gift on the paperwork - unfortunately I don't feel comfortable falsifying customs documents for my ebay sales. I will mark it as it is - a purchase (or whatever it's called) and will claim the amount you paid $xxxx (winning bid only - do not include shipping charge!).
I thank you for allowing me to run my ebay business with honesty and integrity. I do feel confident that you will be pleased with your purchase from me! Please let me know when it arrives - either via email or by leaving feedback. I leave feedback once I know the transaction is complete!
Thanks again!
seller xxxx
xxxxxx auctions