Does the process of selling a computer device with some parts that I don't have in stock, and which will be purchased after an agreement with the customer and the payment of a partial amount, fall under the category of "selling what one does not possess"?

If the contract between you and the buyer has been concluded, and the commodity was not entirely or partially in your possession at the time of the contract, then the contract is invalid because you sold what you do not own. The permissible form is a sale based on a promise, with both parties having the option to complete or not complete the contract.