Look on the face of it, its a great idea.  I have seen dozens and dozens of posts on here and other sites with people asking to exchange gift cards for BTC.
The issue is your website whilst clean is very basic. Lack of contact details/information/ FAQ's - it just smells like a scam. I am sure you are trying to be a legit operation, but right now the scam klaxon is sounding.

10% mark up is reasonable although not amazing.

 I would probably look for alternatives than using you based on all the above.