Microtransactions in Rust are summarized as...

Rust includes a microtransactions store that largely consists of community made skins. These skins rotate on a weekly basis, but can still be acquired through different means.

Skins may be acquired through normal play and are granted at random playtime intervals. These can be sold/bought via the Steam Marketplace.

Some servers may also include additional purchases, including pay-to-win elements. These are all community made servers.

Contains cosmetics for extraordinarily high prices, sometimes costing more than full priced games.
Includes a limited-time always rotating store with exclusive discounts and/or cosmetics.
Contains user generated content or servers that may contains additional microtransactions.
Features a marketplace with community set pricing. You can buy/sell in-game items from other players using real life money.
The original purchase contains all future content, but there are some small optional cosmetics and/or services you can purchase.