I would expect a dealer with this amount of stock to be associated with either AFTAL or UACC too, I know these still don't 100% guarantee anything but do limit the risk.
I've only ever brought from two dealers (sportsignings and golfographs) that aren't associated with these sites and that's because I've met them at several different events!
Absolutely not; avoid like the plague!
Formerly known as 'Cheviotsbooks' and 'The Bargain King' (which by itself should tell you everything you need to know), Jason Cullen was kicked off eBay years ago for selling forgeries by the truckload.
Apart from the vast number of dodgy autographs on his multiple websites Jason now certifies anything as genuine for a fee. His certificates are suitable only for the nearest shredder.
His George Michael forgeries are laughable.
As are those "Bowie's", apart from the Genesis books.