You can't withdraw bonus bets or bet credits, but you can withdraw winnings. Bet credits and bonus bets cannot be withdrawn, but they can be used to wager on other sporting markets your sportsbook has to offer. Some sportsbooks make you use your bonus funds or bet credits in one lump sum.
The short answer is yes. However, not all online sports betting sites are created equal and you can definitely find yourself losing more than just a few bucks. Safety and security checks should be your number one priority before placing any bets online.