To be fair, I placed an offer on an item in which only 100 exist (not comic-related), earlier this year. I couldn't pay immediately and I also couldn't pass on the opportunity to reach an agreement with the risk of someone else finding the listing. After he accepted the final offer, I messaged the seller and told him, "I get paid in 2 days". He told me "NP" and that he'd shipped within 24 hours of payment.
Does that make a difference, if the perspective buyer messages you that it'll be a couple of days?
As has been said - communication is key.