Buying recycled parts isnt a good idea, unless your garage sources them for you and can vouch for their quality.
Avoid scrapyards and reclaimed crash parts at all costs.
Not all second-hand parts are dangerous, however. Replacement parts such as gearboxes, clutches, starters and alternators are often re-manufactured, which means they have been stripped back to their core and cleaned, with all wearing components replaced. These components sometimes make more financial sense than new ones because they can cost around a third less than a new part.
Professionally remanufactured components can often perform better than the originals, too, because any original design faults are engineered out. Always ask your garages advice before you buy.