When tabs are on top options is on; if I click a link (which is set to open in new window by web site or addon) the new opened page's tabs goes beyond the screen limit. when i restore down and restore up the page it gets its normal form.

Question

http://i55.tinypic.com/25jbejm.png ( this is the problem )
http://i55.tinypic.com/303k3kx.png ( this is how it should be)

Answer 1

I don't even know if that's possible, and I have so far not seen any unusual activity on the computer.
No, it's not even remotely possible. What you encountered was an attempt to scam you, and a pathetic one at that.
Identity theft in general is a significant threat to any computer (a term encompassing Macs, PCs, iPads, iPhones...) user. However, that threat most commonly arises in the form of "phishing" and you didn't play along with it. The likely reason the email appeared to have come from one of your previous eBay vendors is that his email address was harvested using any number of methods. The email probably didn't even originate with him. eBay is a rich source for obtaining such information.
eBay has a fairly helpful guide on the subject: http://pages.ebay.com/help/account/recognizing-spoof.html
To answer your specific questions:
Is there any malware or other harmful/malicious computer code--read, something that could steal my identity if/when I do my taxes online this year--currently known to be a threat to home non-business Mac computers?
No.
So, in summary: Should I be worried that I fell for some new (since 2013) method of unknowingly downloading malicious software to my Mac?
No — although Mac-targeted adware has arisen since that time and is a rapidly growing annoyance that can make using your Mac practically impossible, it will not directly cause information theft as you have described it concerns you. Furthermore, it requires your participation to install it in the form of an Admin name and password — and it requires deception to convince you to supply those credentials. Learn more about how that happens here: How to install adware.