I think the one thing that perhaps I haven't stated, and that should be mentioned is the idea of people asking about pieces/parts that can potentially turn a "non-firearm" into a "firearm".

If someone who is obviously new to airguns/guns is asking questions about pieces/parts that will allow for such a thing- any seller of products gets put into an interesting legal position. It is not a vendor's job to tell people how to construct firearms out of non-firearms. When people who are apparenly not completely "new" to airguns pr firearms laws ask such questions, they are advised that making use of the parts they are inquiring about may make them a "firearm" and they are reminded appropriately of their legal requirements/obligations if they use such parts in specific manner.

When someone who is not, at least in as far as can be assessed at first glance, familiar with airguns or gun law- 'educating' them is beyond a vendor's scope, generally speaking. This is where the internet and online forums become useful to people- researching a topic, learning the laws and then asking questions. If I respond to a question with "cut the hammerspring down 6 coils to stay below 500fps and not create a firearm" and am then confronted with the question of "what has been done to this part to produce more power, and how much more power" thereby indicating the inference of "this will create a firearm if you do not cut the hammer spring" has been completely missed or overlooked intentionally is not a good sign- and you have pretty much guaranteed yourself to not getting a response beyond that.

If anything, any seller is more inclined to take a position of either advising to not do anything to modify a product, or discouraging the purchase of parts to someone making it obvious as to their intent of creating 'firearms' where one did not exist previously if they have not established they understand the legal ramifications of such an activity and/or have the appropriate license/permit to do so. No one is going to risk their business by doing something outright and plainly obvious in respect to directly advising someone how to make a product they seem to be, at least on the surface, not legally able to own. It is this initial judgment of a person based on their request(s) that dictates further communications. If someone makes it obvious as to their intent and has not provided any qualification as to their legal status in doing such a thing- the request/question gets trashbinned, plain and simple.

Call it whatever you like, but in my view a vendor advising a person on a product where they have not established this person has any knowledge of the items, or any legal knowledge of things regarding changes of classification in respect to "firearm" vs. "non-firearm" is bad news. Ignoring or otherwise avoiding answering such questions from such people is really the safest way to avoid potential legal issues in the future.