My understanding is that if you legally purchased the knives, you can legally sell them ,customized or not. The only thing that you have to be carefully about is using a registered name of the knives in your business name. You can call it "Joe's Custom Knife Shop", with no problems. But, you can't call it "Joe's Custom Buck Knife Shop".
Listing the available knives by name is fine - "Customized Buck 101, by Joe Smith" or just "Customized Buck 101".
To become a licensed dealer for the company, send them an email. You might be surprised to find out how much knife companies want dealers. That way you could use the phrase, "Licensed Dealer", in your adds.