I personally wouldn't have an issue with it if I was a seller. Sometimes you want something now, and if circumstances outside your control prevent you from getting it when you needed it, it's understandable to move on. I didn't have any pending offers before the site went down, but if I did, I'd fully understand if the site went back online soon and I got a PM that said "sorry, I didn't know if/when the site would ever come back online, so I went with another option instead, sorry". Just make sure you send them a PM that says something to that effect, any decent seller will understand I think.