You want to be really careful accepting the word of a consignor. They have good motive to lie and they have nothing to lose, whereas your reputation and your ebay account are at risk.
In the future, unless you purchased an item yourself from a legit department store or designers' factory outlet, it's a cheap investment to spend $5-$6 to have items professionally authenticated.
There are ebay/paypal approved authenticators whose findings are accepted as proof of authenticity (or un-authenticity). Adding a sentence to your listing that says, "this item has been authenticated by _________" adds credibility to your item.
As for how we know the items are fake, in your case, these items were easy calls, i.e., they were obvious bad fakes.
Without going into a lot of detail (since counterfeiters always have their eyes open to make a better fake), here are a couple of indicators:
http://cgi.ebay.com/COACH-Hobo-Hand...emQQptZUS_CSA_WH_Handbags?hash=item1c0eac10eb -- check out some
Coach listings and look at the creeds. (The creed is the leather patch inside the bag.) You will never see an authentic bag with a serial number that looks like the serial number on this bag. You will also never see a creed with the stamping uneven and off-center like this.
http://cgi.ebay.com/COACH-XL-Tote-B...emQQptZUS_CSA_WH_Handbags?hash=item1c0eac52be -- This is what style 10619 should look like:
http://shop.ebay.com/?_from=R40&_trksid=m38&_nkw=coach+10619
This listing:
http://cgi.ebay.com/Signiture-COACH...emQQptZUS_CSA_WH_Handbags?hash=item1c0eabcbab
This is what 8F34 should look like: