Most reputable companies would not be buying their equipment from Ebay. As an installer anything we supply and install is provided with a warranty, if it goes wrong we have to come back and replace the bits that fail, which is expensive. Therefore it is very risky buying stuff from an unknown source like Ebay. You could be buying refurbished, or repaired, or returned equipment.
I had a customer that used to supply their own batteries every year for a wireless system that we maintained, because they were £1.50 cheaper on ebay than the ones we supplied. That was until they started giving him low battery faults the day before he was due to go on holiday, less than six months after we had replaced them for him. He ended up paying far more, a call-out fee and our batteries at £1.50 ea more (buy cheap buy twice). Our batteries are well within their shelf life from a reputable supplier and warranted by us, not fake copies or old stock. Sometimes buying stuff on ebay can save cash, but I would not buy stuff that I needed to rely on from Ebay .