Does selling a part and estimating the cost of repair before purchase fall under "selling what I do not possess," and is there any objection to it? And is there any objection to not informing the device owner of the available part's price and its installation, then requesting the total cost?

If you agree with the device owner on the cost of your labor, and then you purchase the required part on his behalf, you are an agent in the purchase, and you buy the part as the property of the device owner, and any surplus in the price belongs to the device owner.

However, if you own the part, or you inform the device owner that its repair will cost a certain amount, and he agrees, then this is a valid contract, and you are not obligated to inform him of the price of the part, because the contract was made for repairing the device, which includes installing a component.