If it's a constant habit with this person it's possible that she's waiting on a paycheque and knows how many days she can leave it before she gets into trouble...so bids on something knowing in x days she'll have some money.
Mind you, if her email is like mine, it's also possible the invoices go wandering in the spam folder without rhyme or reason. I almost lost a scholarship offer for my studies because of my email's random penchant for screening out email I NEED, and automated ebay messages are a likely target. If it always happens, though, I suspect it's more likely she's disorganised and only gets around to it when she is nudged. Not great, but at least she does pay in the end I suppose?
As for the multiple IDs, it's actually fine to have more than one ID on ebay these days. I have a seller account and a buyer account and Ebay not only knows this but actually has used one as a security question for the other on at least one occasion. The only caveat to that is that they must never be seen to cross over. So you mustn't obviously bid on your own auctions (lol) but it's also better to keep them totally separate from each other. So long as she isn't doing that, having more than one ID is allowed.
I'm generally of the pov that if a buyer does ultimately pay, then fine, I'll probably let it go. Things happen in real life. It's not the best behaviour, but at least you did get your money.