They are hard to get as no one is willig to sell it, but not really rare as DCE made a few hundred of them (got one myself).
From looking at the picture I can tell that this card is overclocked if it really runs at 50MHz. The place where the PGA 68030 should be is empty, but the place where there is only a printed PLCC68030 on my card seems to be populated with a chip on the picture. Cards with a PLCC 68030 only ran at 33 or 40MHz according to the manual (there is no 50MHz PLCC 030 anyway). That would explain the fan that is part of the auktion.