One login, many economies
A Roblox account is a key to thousands of separate games, and each of them runs its own economy with its own currency and its own rules for what can change hands. Nothing is shared between them by default. That is why Roblox item listings listings name the experience first, and why a price that looks high in one game is unremarkable in another.
The exception is the avatar inventory. Limited accessories sit on the platform rather than inside a game, which is what lets them hold value across updates that reset an in-game economy overnight. Accounts with a deep inventory of those are the listings in Roblox accounts that keep their price longest.
If you are buying to spend rather than to collect, a per-unit order from Roblox currency listings is the shorter route. Either way the escrow flow is the same one described in the Eloboost24 buying guide.