Heya, in addition to what everyone else has said - make sure when copy/pasting your BTC address and giving it to your client, that they are the same address.

There are things called 'clipper' viruses, that when you paste a Bitcoin address, a foreign BTC address will get pasted instead of your own, and funds might get transferred to them instead of you. If your PC is clean without any viruses, there should be no problem, but I like to take note of the first and last few digits of the address to make sure that they are the same.

Also, when you see an incoming transaction on Electrum, right click it and click "View in Block Explorer"

https://i.gyazo.com/0360ac16c8e56279ab7bd79b453626ee.png

Something like this should open in your browser:

https://i.gyazo.com/74014479c3fa5f450d8834cfd3fec544.png

Where it says 'Status', if the payment was just made, it will probably say 0 confirmations. Wait until it has confirmed at least once (this can take a few minutes, an hour, a day - it depends on how congested the BTC network is and how high of a transaction fee he/she paid with)

If this is a person who you trust then you can give whatever it is you're selling him after payment on 0 confirmations (not recommended though)

The reason being is that when a transaction hasn't been confirmed yet, there is potential for it to be 'double spent'. Without getting too technical, and explaining as best I can, double spending is when someone makes use of the 'replace by fee' feature - they essentially overwrite the transaction with another transaction, change the delivery wallet address in doing so, and they put a substantially higher fee. This way, the higher fee ensures that the block is mined before the original one, and the 'overwrite' of the transaction gets confirmed, and the old one (to your address) doesn't, and you don't get your bitcoin.

So usually just stay safe with the 1 confirmation Tongue