Getting the stamp on the 20 euro electronics purchase is in the customer's best interest as it is a proof that he purchased the item there and so can get warranty service from the seller (I don't know if "warranty" is the correct term here, in German we have two words for two concepts that wikipedia both links to the English "warranty" and that most people confuse anyway: "Garantie" (cognate to "warranty") which is a voluntary service offered by the producer, and "Gewährleistung", which is a legally mandated service by the seller. The stamp documents your right to get "Gewährleistung" from the seller who puts his stamp on your receipt, and usually the producer's "Garantie" requires that stamp, too).
It is more difficult to forge a stamped receipt, true. As to whether it proves that you purchased the item is debatable because both a receipt and a stamped receipt can be easily forged. The question is: is it necessary? Tally up all the man-hours a country spends stamping receipts. Could that time be put to better use? North America seems to function fine without them. Perhaps we've come to realize them as redundant - or are we too gullible to not demand rubber stamped receipts in the first place?
