Bargains at online auction houses, from „Friends“ or flea markets can be properly „expensive“.
If you buy a new or reconditioned product at a fraction of the usual price, you can, or rather should assume that it is stolen (e.g. a new computer for 80 € instead of 800 €). That makes you a committing a crime for receiving stolen goods (§ 259 Penal Code).
Particularly significant here is the punishment of up to five years.
For your understanding, here’s the legal text:
§ 259 Receiving stolen goods
Who buys or appropriates an object that another one has stolen or otherwise acquired by a directed unlawful act against property, for himself or a third party, sells or helps selling it to enrich or enrich a third party is punished with imprisonment up to five years or a forfeit.
(…)
Incidentally, you can not acquire ownership of stolen goods. This is regulated by § 935 BGB. The rightful owner may in any case require back his object, regardless of what you have paid for it. You have indeed a claim against the person who has sold you the stolen object. But such a claim is rarely enforced because sometimes it is difficult to locate the seller (on eBay things are often sold under a false name) or this person has no assets, so it is difficult to enforce an existing claim