Oh I'm sorry I was not aware of that. Can I still sell them on the exchange ?
Essentially, if you make money from modifying knives, you need a knife maker membership. If you are doing it purely for the love of doing it and there is no monetary return in it, you don't need a knife maker membership.
Allow me to clarify.
You modify a knife for your own personal enjoyment and don't modify knives for others for a fee. You can display your knife if you want to at any membership level.
You have a knife that you modified for your own enjoyment and you don't modify knives for others for a fee. If you sell the knife, Gold membership covers it.
(That works for a knife or two. If the mods see you selling a lot of modified knives, you are going to need that knifemaker / craftsman membership.)
On the other hand:
If you modify knives for others for a fee, then you need a knife maker / service provider membership in order to display knives that you modify. even if they are your own.
If you modify knives with the intent of customizing them to increase their value so you can sell them for more money, then you need a knifemaker / service provider membership. (If you sell a lot of customized knives, the mods are likely going to require you to get that knifemaker membership, no matter what your stated intentions are.)
And you need a knife maker / service provider membership if you are going to advertise your services in your signature line. Ahem....