How do you explain to the shop owner about waiting 20 minutes for a confirmation?
If you understood Bitcoin, you would know that purchases of under 25 bitcoin (number of bitcoins created when a block is mined) make it impractical for someone to strategically produce a 'double spend'. As if someone was to 'double spend' they would have to solve a block (and have the 25 bitcoins for that block ready to obtain), then hold onto that block without broadcasting it, make a transaction, ignore that transaction with their computer that has mined the block, then after the purchase submit the block, which would 'fork' the blockchain with all transactions but the one they made, therefore keeping the ownership of their bit on the ledger (bitcoin). This would all have to be done within 10 mins (finding the block, making the purchase, confirming the purchase and walking away). If they failed the double spend due to someone else solving the block, they would lose 25 btc
The hassle, inconvenience and risk of attempting this would out-way the purchase of under 25 btc 1000 fold. If you were to buy something worth, say, 10 000 USD + then you could wait the 10 mins for one confirmation, then it is exponential from there, so if the transaction was over lets say 100 000 USD you'd wait for 2 confirmations. I think this only applies to Brick and Mortar purchases, e-commerce would have slightly different considerations. Also, 1 in a million attempts at double-spends that might happen (and someone gets away with $2) would be minute compared to charge-backs from credit card companies today.
