Proper way to sell digital codes question?

[HDMan72]

Questions regarding selling codes. I list the codes for sale, I dont sit waiting for PM's while staring at my computer. I'll check maybe 30 minutes later. Now I have 5 potential buyers lets say. I message the first buyer, seems right to me. I send them payment info and wait to receive said payment. Once I get it I mark the item sold and contact other buyers and let them know item(s) sold. this usually happens within 20 minutes. Should I be doing it another way? I just received a message accusing me of waiting it out to see who will buy it from a person who was 4th on the buy list.

Just looking to see if I should be selling differently, easier method maybe?