This is my field. If you are at the point of asking where you can buy parts in wholesale, then you are most certainly not in a position to be opening a car parts business.
Forget eBay. Type in any part on eBay - brake pads, a wiper blade, trim clip, cleaning products, whatever you want. Look how many thousands/hundreds of thousands of results there are. From the established guys that have been on eBay for years, to the likes of GSF and Motor World who now use eBay as their main e-commerce site, and the Chinese factories that are supplying directly to end users at rock bottom prices. Where do you want to fit in within all of that?
If you want to supply locally, have you a couple of million to sink into stock? You may get some suppliers to supply you with impress stock but you'd have to demonstrate some serious experience/knowledge/forecast figures. The UK parts market has moved towards such rapid turnarounds that nobody waits for anything any more - every single SKU within a programme, whether it's the set of pads you sell once a day or the set you sell once a year, you'd be expected to have in stock. Garages are getting deliveries from factors up to 5 times daily. Whether it's an exhaust, a full service pack or a single o-ring for 50p, they can call the factor and get it delivered usually within the hour. Not available within the hour? The factor can often get it before the end of the day or the very next morning at the latest.... Can you compete with that?
ECP will now supply to anyone and everyone, whether you're a garage spending £1000 a day or Joe Bloggs walking in off the street - click and collect within the hour and pay more or less the same price as the garage. If you did manage to make some impact locally they could kill you within a month if they decided to turn aggressive - and it wouldn't be the first time they'd done it.
Some independent retail stores (as oppose to factors) are continuing to thrive but for a number of reasons - usually with the knowledge and experience to dig out or find unusual parts, or go "the extra mile" whereas ECP will just look on the computer system and if a part isn't listed, forget it. But on top of that most of them have branched out into cycles, key cutting, vaping/e-liquids, and mobile phone repairs.
It's a tough, tough business being swallowed up and overrun by major players - even many branches that appear independent have been bought out and are in fact owned by one of the major buying groups.