Understand basics like the following:
1. Learn to back up your wallet balance
2. Learn to save your private keys and do not share them
3. Learn bitcoin balance typo errors can be made eg: 0.1 when you meant 0.01
4. Learn bitcoin is a non reversible payment
5. Learn to use reputable escrow with +1 or higher
6. Know in the bitcoin world of business anything can be gone within in instant.
Is backing your btc wallet really necessary?
I suppose, it is unnecessary if I just buy/sell. is it right?
Yes if you want to keep your bitcoins online without having a wallet on your computer, but I don't know if that is very safe.
If the site goes down or is hacked, you could lose control of the bitcoins. : /