I'm guessing the OP is a touch on the young side and the comments he's getting are from parents. I'm going to make that assumption in my response to this post. If the assumption is incorrect, then please don't take offense for it.
What I would suggest is that you have a good discussion with your parents and perhaps a bit more discussion with your father about why you are considering something like a Sebenza for purchase. Especially, if your B-day money was provided largely by them and or other family, they want to see you make responsible decisions and wise choices for resources they worked hard in order to secure for you. While the money is yours and given to you, it still is a respectful consideration to involve them in your purchases, your choices for such purposes and to share your excitement and gratitude for the gift you received.
Frankly, spending $350 on knife if neither one understands its value or why it is valued as such is a poor demonstration of your respect for them. Imagine, how it long it might take them to secure that money. Consider how often your father or mother makes a splurge spend of that kind of cash on themselves.
If you can't get one of them all that enthusiastic about your purchase even after explaining to them its merits and value then I suggest going at it in a different tact. I suggest asking them to use a portion of the money, lets say half the value for the knife, for the purchase and then come up with an alternative plan to save up for the remaining portion of the purchase. This could be something like taking on a paper route, cutting grasses or offering to rake leaves in the fall. Any form of legal part time labor.
Here is the kicker, if you parents see that you are motivated to earn you way into this purchase, they will start to affiliate your interest in cutlery as a positive thing in your life rather than a negative thing. Maybe you can even convert one of them into a person he sees the value of cutlery.
Hope this helps, and trying to let you see things from a parents eyes
p.s. I think it is great that you chose not to lie to your family when they asked you about your intentions - that shows good maturity and respect already.
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