Personally I would trust most of the biggest German gold dealers right off the bat. It is far better business to sell gold with honest business practices, than try to pull a quickie. Most of the ingots arrive directly from the manufacturer and come in plastic & serial numbered.
But if trust is still an issue, why not buy gold directly from mint? Heraeus moves 2 billion EUR gold/year, has an online shop, and one can go pick up the gold from Ophirum's shops.