So heres what happen. Someone wanted to buy from me and ask me "how long it will take for me to deliver", he/she keeps asking me that question, until he says he sent the bitcoin. He/she gave me a transaction link where i can see the transaction still unconfirmed he still asking me how long it will take for me to deliver, i told him/her that he can check the link he/she provided. After a few minutes i refresh the link and says "INVALID".

Just wanted to share because this is the first time i had this case. I dont even know that they can do that on bitcoin... kinda new on bitcoin