How would I go about sending a Webhook when a user uploads a clothing item or changes the price of an item in a group?
I could be incorrect. You may need to actively watch the price of the asset in order to actually see it change via a external script outside of roblox(or within roblox if thats possible, I would research that if I were you. → MarketplaceService | Documentation - Roblox Creator Hub).
I personally would have created my own API for this via Python of JavaScript.
I don’t believe there is literally a clothing_price.Change:Connect. You just have to implement it yourself via scripts or external services/API’s