One of my friends toyed with an idea like this, basically it used something similar to SWEP Construction Kit to edit weapons(actually, it used SWEP Construction Kit entirely), and if a player had a certain skin unlocked and selected, it would use SetMaterial to change the skin. Just using the default materials in GMod you can make some pretty neat stuff. It was very unfinished however and the only remnants that I have are some bits of code for an early drop/reward system and a table of materials that don’t look horrendous when applied to weapons.
Skins could be applied to any weapon without having to make a thousand different identical SWEPs like mentioned earlier. It even included varying skin rarities, and some of the animated textures like the pulsating red/green/blue lines from PHX look really cool, you could even go as far as make your own animated skins if you bothered to mess around with VTFEdit.
personally if I finished this I wouldn’t bother with a trading system or mini economy as there’d more than likely be a way to exploit it and it’s just overly complicated. Neat skins would give players a reason to keep playing your server, make your gamemode more unique as opposed to just the same pointshop literally every server has except slightly modified, and your chat won’t be spammed with “admin this guy just scammed me !!!”.