user1475: I have an online wallet at localbitcoins.com (address 12pHFR5yv2KGqck5N8teb5MgoRtzAgLRn2).
I made three transfers to it, for a total of 0.5 BTC + 0.5 BTC + 0.3031 BTC = 1.3031 BTC.
Blockchain.info did show my address receiving coins. However, it is showing my address sending coins to several addresses after receiving them https://blockchain.info/address/12pHFR5yv2KGqck5N8teb5MgoRtzAgLRn2:

Is someone using my online wallet at localbitcoins.com to withdraw money?

user516: Probably not. Most online wallets don't hold your bitcoins against the receiving address for your wallet; instead they transfer them to a central wallet and keep a record of your balance.
When you want to spend from your wallet, they make a transaction from their centralised wallet to your desired sending address, and adjust your balance accordingly.
This method of providing an online wallet has the advantage that they can reduce transaction fees (they can spend their oldest bitcoins first, and consolidate appropriately sized inputs for your desired transaction output), but also some disadvantages such as that you don't get to hold the private key for your receiving address (and thus can't retrieve your bitcoins without them).

user153: You do not have a Bitcoin wallet at LocalBitcoins.  Instead you have only a ledger account with their hosted (shared) Bitcoin E-Wallet.  
What this means is that you cannot use the blockchain to track your balance.  You can only use it to determine how many confirmations your transfer (deposit) has received.
The E-Wallet provider simply credits your account when they see a confirmed deposit transaction (e.g., 6 block confirmations) to a Bitcoin address associated with your E-Wallet account.
To know your balance, you will need to login to your E-Wallet account on LocalBitcoins.  That will show you your balance.  The account details will show deposits, withdrawals, purchases and sales.

1475: <p>I have an online wallet at localbitcoins.com (address 12pHFR5yv2KGqck5N8teb5MgoRtzAgLRn2).</p>

<p>I made three transfers to it, for a total of 0.5 BTC + 0.5 BTC + 0.3031 BTC = 1.3031 BTC.</p>

<p>Blockchain.info did show my address receiving coins. However, it is showing my address sending coins to several addresses after receiving them <a href="https://blockchain.info/address/12pHFR5yv2KGqck5N8teb5MgoRtzAgLRn2" rel="nofollow noreferrer">https://blockchain.info/address/12pHFR5yv2KGqck5N8teb5MgoRtzAgLRn2</a>:</p>

<p><img src="https://i.stack.imgur.com/RuDjH.png" alt="enter image description here"></p>

<p>Is someone using my online wallet at localbitcoins.com to withdraw money?</p>
 516: <p>Probably not. Most online wallets don't hold your bitcoins against the receiving address for your wallet; instead they transfer them to a central wallet and keep a record of your balance.</p>

<p>When you want to spend from your wallet, they make a transaction from their centralised wallet to your desired sending address, and adjust your balance accordingly.</p>

<p>This method of providing an online wallet has the advantage that they can reduce transaction fees (they can spend their oldest bitcoins first, and consolidate appropriately sized inputs for your desired transaction output), but also some disadvantages such as that you don't get to hold the private key for your receiving address (and thus can't retrieve your bitcoins without them).</p>
 153: <p>You do not have a Bitcoin wallet at LocalBitcoins.  Instead you have only a ledger <strong>account</strong> with their hosted (shared) Bitcoin <strong>E-Wallet</strong>.  </p>

<p>What this means is that you cannot use the blockchain to track your balance.  You can only use it to determine how many confirmations your transfer (deposit) has received.</p>

<p>The E-Wallet provider simply credits your account when they see a confirmed deposit transaction (e.g., 6 block confirmations) to a Bitcoin address associated with your E-Wallet account.</p>

<p>To know your balance, you will need to login to your E-Wallet account on LocalBitcoins.  That will show you your balance.  The account details will show deposits, withdrawals, purchases and sales.</p>
