Accidentally flaked on my [19F] friend [21F]

I'm a part of has a Facebook group for a student club, and since it's football season, people are selling their football tickets. In the first week of October a casual friend [21F] (whom I know just because we share a friend group) posted asking if anyone had two tickets for a game on Nov. 4th. I knew I wasn't able to go that date so I offered my one to her, and she 'liked' my post so I knew she had seen it.

 
However, she apparently also Facebook messaged me that time asking for my phone number, which I didn't see for a week because a) Facebook doesn't alert me to messages and b) we saw/interacted with each other everyday and I assumed she could talk to me. When I saw her message I immediately apologized, and said I don't get alerts, but here's my phone number. Several days passed and I hadn't heard from her (she had sent me no more messages since--I checked) but I saw that she had claimed tickets from two other people. I figured that's fair, I didn't respond so she bought someone else's and she got the tickets she needed. So I sold to someone else [20F] (by posting on fb), and they immediately paid me for the ticket. Hindsight is 20/20 and I should have clarified with the original girl that she no longer wanted/needed my ticket. I thought that was the end of it but I finally log into facebook today on my laptop and she's sent a message saying "Wait! I thought I was buying your ticket??" That was sent on the 19th, the day after I sold the ticket.

 
I feel absolutely guilty and anxious because I HATE when others don't treat people the way they want to be treated. I feel really bad and I don't know what to say other than "I'm so sorry for the bad communication, I never got these messages and I made a mistake in assuming you had bought the other tickets when I responded." What else can I say/do? Should I apologize in person or over text?? Idk if I'll see her over the weekend, but she'll be able to see that I've read her message and waiting longer makes this look worse...

tl;dr I promised a ticket to a friend, but due to a bad mode of communication I sold it to someone else, and the friend thinks I'm avoiding her because I flaked. What do I do to apologize?