Can perpetual bonds be sold?

Excuse me, but I was wondering if you could clarify something for me. The concept of perpetual bonds has always intrigued me, but I'm still a bit unclear on their liquidity. So, my question is, can perpetual bonds actually be sold? I understand they don't have a maturity date, but does that mean they're completely illiquid and untradable? Or is there a secondary market where investors can buy and sell perpetual bonds, just like with other types of bonds? I'd appreciate any insight you can offer on this matter.