Site sponsor TradeEx has got great deals on a bunch of older European bird guns, as you have found. I honestly don't know where they got them all!
You need to read the description for each one you're interested in, because some do have minor problems, but importantly, not all have 2-3/4" (70mm) chambers for using common modern shells. Apart from any that might note missing hammers, broken firing pins, and so on, they should all be functional guns.
As for makes, many are of the brand JABS (Just Another Belgian Shotgun) or AOSS (Any Old Spanish Shotgun). The point being, there have been many factories making these through the years, and many different brands attached to guns from the same factories. The guns I have seen there represent really good value in the market, with a wide selection and lower prices than you're likely to see elsewhere for the same, but they're not just putting out a crate full o' guns and all at one price; the cheaper ones listed are generally not as good as the higher ones.