I was wondering if I buy a strange killstreak weapon from the Community Market, will it reset the kill counter it already has on it, to 0?
Yes, it will. This applies to any Strange weapon or cosmetic item, and to Steam Community Market purchases, regular trades, and gift trades (i.e. Gift Wrapping the Strange item before sending it to the recipient).
The notes on the wiki say that simply listing a Strange item on the market will reset its kill counter to 0. I haven't sold anything on the market before so I don't know if it's true, but this is something to keep in mind if you plan on selling.