Yes, it really matters that much. Sounds like you need a "Medium" if you go for a Giant. The TCR has an "M/L", but even that is probably too big for you. (I'm a 5'9" male and the M/L TCR was a bit too big for me.) Test ride at your LBS and make sure. Ride some other brands too, before you make a decision. That will probably mean visiting more than one LBS. Ride a Trek, a LeMond, etc. Talk to the staff...the one's with experience, that is.
Regarding eBay, be aware of pervasive fraud in bike listings....things like "second chance" offers from people other than the person with the initial listing. Having said that, I and two of my sons have bought bikes via eBay, but only one involved purchase and shipping; the others we picked up directly from the seller. There are lots of bike listings on eBay. Try doing a search for bikes within 50 miles or so of your address. (Setting is on left side of the screen.)
I recently identified three local bikes from different nearby eBayers, test rode all three, checked their condition, then bought my Aegis Aro Svelte; got a great deal with no chance of fraud, which given what bikes cost is a real concern. Plus no shipping. If the bike of your dreams isn't listed today, look again tomorrow. They're like busses....if you miss one, another will be along soon. Or.....if you like what your LBS has to offer, then purchase from them.....but get a bike that's right for you!