Don't just blindly trust your auto mechanic if he recommends spare replacement. The person at work may be skilled, but he need not be honest. If you still aren't convinced, the following few reasons will offer some validation.
Asking for old spares helps ensure the installed parts are indeed new and aren't the refurbished versions of your earlier components.
When your old auto parts are returned to you, insist on having them packed in the box of the new components. Having the fresh box means you can cross-check it to confirm the brand authenticity (genuine or non-original equipment), the date of manufacturing (not an old stock), warranty details, etc.
Moreover, those worn-out parts are your property, and you are entitled to have them in possession. When the mechanic gives the parts back, you could always take them to other auto repair stores and find out whether they really needed replacement. This also helps confirm that the parts belong to your vehicle and weren't random components passed across.
When seeking the parts, ask the mechanic which part of the component broke and how it all happened. If it's possible, request to have a live view of the service in progress.
If the original mechanic hesitates or seems reluctant to hand over the older parts, you can pretty much tell how trustworthy his service will end up being. A reliable auto repair expert is one who is forthcoming to the customer's requests and has no qualms about taking the owner to his vehicle to show what is being done. If the mechanic doesn't degrade or demean your knowledge in the domain, he is one for the keeps.
If you relinquish your old parts to the mechanic and if those aren't as worn-out or unusable as claimed by the mechanic, he may fix your old spares and use them as new when another customer comes by. Possibly you may end up that unlucky person again.
And if the shop is in to selling second-hand parts, your used vehicle components will be added to their list of products for sale.
Some basic knowledge about your vehicle is essential as that helps you gauge your vehicle's basic structure and requirements and forecast when it'll likely tear apart. Besides, this fundamental awareness keeps you in good stead when interacting with the mechanic to interpret his suggestions – quite handy if he's trying to oversell or recommend unnecessary services.
As a practice of caution, always check your vehicle's warranty status. If your vehicle hasn't yet crossed the boundary, take it to an authorised dealer or repair facility. You are unlikely to be scammed at authorised shops as they risk losing their dealership status when suspected of foul play.
For more information, contact a business such as About Auto Mechanics.