How to buy monero for cash ? I meet with the seller, then give him cash and send him my wallet address using some messenger. Then he using a laptop or smartphone send xmr to my address. How much time it will take until I will see in my smartphone that I received monero ? What more I need to know ?
When the seller initiates the transaction, the transaction will be broadcast and relayed to all Monero nodes immediately. All well connected Monero nodes should see the transaction in just a few seconds.
Depending on your specific wallet software, you may see an immediate notification that payment has been initiated to your wallet. You will see the full details of the payment, including the amount.
If you were to accept the payment at this point, there is a risk the seller may attempt a double-spend. This means the seller could announce a competing transaction which spends the funds back to the seller's own account. If the amount is small and the seller has a reputation to uphold, this is unlikely. But it is possible. Your risk here is similar to accepting a $20 bill from someone without taking the time to verify that it is counterfeit. Someone in a dark alley is more likely to attempt to defraud you than a shop keeper whom you can easily confront later.
If you want to be safe, you'll want to wait until the transaction gets mined into the blockchain.
Monero blocks are mined every 2 minutes, on average. Therefore, you will have to first wait an average of 1 minute before any block currently being mined has finished being mined. Then, you'll have to wait on average 2 minutes before your transaction gets mined into the next block.
If the network is busy and the seller's transaction did not have a high enough transaction fee to make it into the next block, you will have to wait until the network gets less busy. Therefore it's a good idea to wait for the transaction to be mined into a block, so you can be sure the fee paid was high enough for this to happen.
At this point, for the seller to defraud you, they would have to run a set of competing miners that would outcompete all existing Monero miners. The objective would be to create an alternate blockchain that ignores the transaction you had originally seen as mined, and replaces it with a transaction that spends the funds back to the seller. This is very expensive, and gets more expensive with the number of blocks in a row that the seller is attempting to outcompete.
It makes no sense for the seller to spend the vast amount of money required to attempt this attack if the amount of cash the seller is accepting from you is small.
At the time of writing, www.crypto51.app reports that it would cost approx $17k per hour to attempt to outmine all existing Monero miners.
Therefore, if you were to wait around with the seller for an hour after the transaction was made, it would make no sense for the seller to defraud you if there was less than $17k at stake.
In summary, if the seller is reputable and honorable, you probably want to wait for the transaction to appear in the blockchain for the first time (a wait of approx. 3 minutes on average). If they're very honorable, just the instant confirmation will be enough for you, because you can approach them later if anything goes wrong and ask them to re-initiate the transaction.
If they're not trustworthy, then you should wait until the cost of a 51% attack is likely to exceed the amount the seller can defraud you for.