I am completely new to bitcoin. I recently received bitcoins from a poker site, using a receiving address that I created in Bitcoin Core. When I realized that synchronization takes forever, I installed Electrum instead and added the receiving address to the default wallet, which is watch only. Now I want to deposit my coins to Neteller but they are on this watch only wallet and I can't make a payment with it. The wallet doesn't seem to have a private or public key (I read somewhere that i might need a private key that I would then import to a new wallet?). What the hell do I do?
You need the private key for the watch only address.
Assuming you've kept the wallet.dat file for Bitcoin core (otherwise you're out of luck), open the console under help in Bitcoin core, then type
dumpprivkey x where
x is your Bitcoin address. You may need to unlock your wallet first with
walletpassphrase XXXX 99 where
XXXX is your password.
The dumped privkey will be in WIF format (5J/K/H... or K/L...).
Use Electrum's console tab to import this key:
importprivkey WIF where
WIF is the WIF key