I submitted my order of selling my unwanted Walmart gift card at about 10:00 on Monday night and already received my payment around 1:00 on Tuesday very impressive

They will take several days to approve your sale and they can choose to randomly cancel your order or sale with no reason given. How long your payment takes to arrive depends on which method you use. Mailing you a check takes several days to arrive in the mail whereas having them give you the money via PayPal will be immediately after your sale is approved.

I received a check in the mail 2 weeks after they received my cards. Other payment options may be faster.