I would consider it very poor practise to re-use secondhand equipment for a paying customer.
The main exception would be in the case of obsolete or no longer readily available parts, under which circumstances re-use is justified if the customer is aware of this and approves it.
Just recently I installed several old light switches obtained from fleabay, the reason being that new switches suitable for 250 volts DC are not available.
I might make the odd exception for virtually new components that have seen very limited use, but not regularly.
I am more relaxed about use of serviceable but used equipment for free/very cheap jobs done for friends, relatives, and charities. Or of course in my own home.
on Friday I fitted a load of old square edge MK stuff I obtained of eBay as the flat owner wanted MK to match the existing MK. I only charged for exactly what the materials cost me and explained they were used seconds but I checked over and all seemed in good working order.
what I really liked was the 3 old style switch spurs had the old wrap around terminals for the load side for the stranded cable.
But as above old stock / seconds are generally used for cheap temporary jobs or mates rates jobs
