Over booking flights is so annoying and such an inconvenience

Today I tried checking in online for my flight to East London and I couldn't all seats were full and I couldn't check-in without selecting a seat. I got to the airport on time way before boarding and I was to that the flight is full and I have to change my ticket I get to the ticket sales they say my ticket is with a third party because it was booked by the travel agent I called them they say my ticket says it has been used I can't re-use that ticket and the agent was annoyed because she had several complaints about the airline. She told me that they over book their flights and when you come its either you are too early or too late you are never on time with flysafair and their flight controller on duty was so unprofessional she left me at the desk while I was talking to her. I'm so annoyed with this airline.