Are they established IDs or guest accounts?
It's possible vouchers were involved, as well. They always have a maximum value per transaction but can be used more than once, so for items around that much each, they could have saved more money by purchasing them separately.
There are often logical reasons for things that look like red flags, and then sometimes the flags just really are red, but it can be impossible at times to tell the difference except in hindsight. A the end of the day only you can decide what you're comfortable with - no reply to messages (through ebay or directly) would make any flags I see raise above half-mast, but again there could be a logical reason for it.
I often just ship things anyway, if it's a risk I'm willing to take, but for higher value orders I tend to err on the side of caution, with the reasoning that if the buyer is on the level and wants to complete the transaction, they'll get in touch sooner or later - though if discount vouchers were involved, they may just be ticked (since they can't be reimbursed / re-used).
How long before the ship-by date? I always include a deadline when contacting buyers about orders I can't / won't ship until I can confirm something - if they know that is one the line, they might be more inclined to respond, at least then (if they respond) you can get a bit more of a feel for the situation.
