There’s no requirement when character trading to be online simultaneously; this is a multi-timezone game, and people frequently post on the forums at times they’re unable to log in. Every other character I’ve bought has been send isk + evemail with account name, receive notification of character transfer. I’m not sure where you got the notion that synchronous availability is required, but if you look through other threads here you’ll not see other people demanding that as a condition for sale.
You’d of been in a rather awkward position if multiple people had sent isk & account names. In any case I’ve bought a different character, without any fuss from the seller about availability.