If I decide to sell something with LN payment, can I use an address that is outside the LN wallet?
No, Lightning Network does not use addresses for payments. Instead, you need to create an invoice. Some implementations already support invoiceless payments, but those are not available in most LN wallets and I am not sure if they are cross-compatible between implementations since this feature is still not a part of the official specifications. Keep in mind that you can't start receiving payments right after you open a channel. I have described it in detail in the FAQ.
In other words, do I pay fees to transfer bitcoins from LN network to,let's say, my Electrum wallet? If it is an off-chain transaction, how to I go back on-chain?
In order to move your coins off-chain, you need to open a channel with some node. That requires you to send an on-chain transaction to a 2-of-2 multisig address controlled by you and the other node. The transaction will be created automatically by your wallet. The channel will become active after the transaction reaches a few confirmations. In order to move the coins back on-chain, you need to close the channel. If the other node is online, the closure should happen instantly and your on-chain address should receive your remaining LN balance minus the closing transaction fee. If the other node is offline, you will have to wait some time before you can broadcast your closing transaction. It's a cheating countermeasure. As you can see, you will pay only for two on-chain transactions and you will be able to make an indefinite number of off-chain transactions.
By the way, if you are interested in using Electrum's implementation of the Lightning Network, check out
this thread.