Permissions, what are the reasons for the general trends

So far I noticed vehicles are all copyiable but not transfer, that makes sense because they get lost so often
Hair. well I guess you really can't give a hairstyle to someone else in RL, but then I also seen transferable skins & shapes?
Guns. I guess it's to keep griefers from just passing them around
But what if someone starts selling non-copy transferable guns, do they get slapped on the wrist by Lindens or just sell less guns?
Why are things like sunglasses & boots sometimes non-transferable.
Is it a general market leader type thing or some old Linden code to abide by?

A lot of/Some people like to put an entire outfit (shape, skin, clothes, accessories, etc) into one single folder. In that case they need to be able to reuse a skin, hair or shoes more than once for different outfits. Copy is a general catch-all for that.

Some people can prefer copy on mod prim attachments as well, since they can work on a copy in case they mess up.

If you don't sort your inventory that way then you don't really have a need for copy and prefer transfer, or you just don't care.

Hair specifically (personal guess really): plenty of stores actually sell hair in packs (more than one colour or style). The pack is priced fairly so that even if you only use one colour/style you're still not overpaying. If you use all of them, you get a lot cheap. If hair was transfer the store would get less sales and be forced to sell one hair at a time, raising prices overall.

The above can extend to more things than just hair: an outfit in one colour costs L$200 but the 5-colour pack costs L$600. A single colour copy might be NC/T, but the pack would likely be C/NT to keep someone from buying the pack and undercutting the seller.

Or an outfit where everything comes on all the different layers or in different versions could be C/NT for no other reason than to keep people from "cheating".

In some cases the permissions you get aren't because they make particular sense for that item, but because they're the result of denying you either copy or transfer.

if you're a seller you have two choices +C-T, -C-T (ok 3 -C-T but I don't buy from those people)

reasoning is as stated above, basically you want to prevent floods of your items from getting copied an passed along, so you can continue to selll things... +T is obviously useful as a gift, +C is obviously useful for creating multiple outfits with the same piece

+M items are generally sold as no transfer to prevent people from duping it and using it in griefing tool with your name as creator...

shoes and other complex items that rely pretty much on shape alone tend to set -M to prevent prim copy scripts from duplicating the item, and the duplicates being sold under a new name. same case for clothes since the item can be seen in the edit appearance window otherwise (although the texture shown is tiny so I'm not sure about it's worth as protection)

I prefer the idea of +M (whenever possible) +C +T and gift cards which covers the giving of an item to others, provides maximum useablity for the buyer/reciever, and allows you to sell one to a person while maintaining flexibility

there are also SOME creators that sell -M because they do not want their work modified out of some (misguided?) sense that their product is superior in every way for every person, and could not posibbly be altered w/out diminishing it
(much like a chef that requires the restaurant not to place or allow condiments on the tables, because it would 'ruin' their creation... never mind that different people have differeng tastes)

I had always had it explained to me that the reason hair is no-trans is that the buyer sometimes needs to modify it to make it fit.

Every hair maker's notecard on fitting hair that I've read highly recommends that you make a copy FIRST and edit the copy so that you have something to fall back on if you mess it up. And - if it's copy, it can't be transfer, or you might as well give it away.

I don't know much about guns, or glasses, or boots. In cases where there is no specific reason to choose one over the other, makers set copy/transfer permissions based on either their own preferences, or the perceived preferences of their customers.

Vehicles _have_ to be copy, because of the number of ways one can lose a rezzed vehicle (sim crossings, entering banned parcels unexpectedly etc). If something is copy it has to be no-transfer unless you want to see your hard work appearing in a business-in-a-box for L$50 and no cash to you.

There's no particular reason that weapons should be sold copy no-trans, but I tend to set them that way these days, because it makes updating a product a lot easier. If a weapon that I sell has a script which checks its version number and sends the owner a new one if the version is an old one, that's fine if it's copy no-trans - that can't be done if it's no-copy trans, I would have to use a much more complicated mechanism.

Also, to be quite frank, half the time people are only buying weapons so that they go with an outfit, and in that case it is more convenient for them to have copiable versions, where they can put them into several different folders for different occasions.