It's possible the buyer has purchased as a guest, which creates an eBay username / profile but it's not an actual account. Some people who shop online also do things like create temporary email addresses, or at least use a throwaway that they rarely - if ever - access, so that they don't provide their regular details and avoid things like spam etc.
Which seems understandable to a degree, but ignores the fact that they might sometimes be required to participate in the transaction beyond just paying. (If you really want to get in touch with them, view the order details and check if a phone number was provided).
My personal view is, that's on them. If they make it so that I can't proceed with their order, then the order gets cancelled, no ifs or buts about it. My first message to a buyer where I need important info from them will include a line advising that if I can't confirm the info by X date (which is when my handling time will be up), the order will be automatically cancelled and refunded, but that they can contact me at any time to discuss further options and / or re-purchase if that occurs.
Funnily enough, though I need to do this several times a year (cancel orders due to lack of info / response), only one buyer over the years has ever contacted me after the cancellation and organised re-purchase, and while I don't sell essential items, some of these orders were relatively high value (compared to my average order), so I'm always a little surprised that I never get "hey, what happened to my order?" messages.
Could it be a scam? Sure, but so could every purchase made on eBay, until it's clear it's not. I tend to think someone who is intending to scam someone else, though, will respond to get the package on the way, reassure the seller, etc, but, you know, so would a genuine buyer so there (usually) isn't any one thing about a buyer that says "this is a scam", rather more it's a series of red flags that tend to suggest it.
