Roblox Game Bot Developer for Marketplace Interaction

The description you quoted is a common explanation of how many player-to-player item marketplaces for Roblox have historically worked, but the exact implementation varies from site to site and isn't public unless the operator discloses it.
At a high level, an automated delivery bot typically works like this:
Customer places an order
The customer logs into the marketplace.
They enter or link their Roblox username.
The marketplace records which virtual item should be delivered.
Order enters a queue
The backend stores the order in a database.
A worker process or bot continuously checks for new orders.
Bot account receives the job
One or more Roblox accounts controlled by the marketplace hold inventory.
The system assigns an available bot that owns the requested item.
Automated in-game actions
Depending on the game's mechanics, the bot joins the game and performs whatever legitimate in-game transfer mechanism the game supports.
In games that have trading, the bot initiates a trade through the game's own trading system.
If the game doesn't support trading, delivery may instead happen through another game-supported mechanic (for example, purchasing from a player's booth, gifting if supported, or another developer-provided system).
Completion
Once the transfer succeeds, the backend marks the order as completed.
The customer receives a notification or sees the status update on the website.
The "bot" itself is usually made up of several pieces:
Website/backend – receives purchases and tracks orders.
Database – stores pending and completed orders.
Automation service – watches for new orders and tells a bot account what to do.
Bot account(s) – Roblox accounts that own inventory and perform the in-game delivery.
Monitoring/logging – detects failures and retries if something goes wrong.

Timeline is ASAP, so please let me know how fast you can deliver a first test build. I’ll share the exact item list the moment we start, and we can iterate from there until trades run smoothly and reliably.

An example of this is seen on the website buyblox.gg

https://www.youtube.com/watch?v=6qSc7JHGX1g&t=2s

This is an example video of exactly what we need