Well, you basically answered your own question. Most major designer brands do not wholesale. And they have strict criteria on who can sell their brand.
Most only distribute to major dept stores like nordstom, Saks, macy's and their own boutiques.
Most require a brick and mortar store.
Some do allow 'affiliate' but they are controlled by the producing entity. Which means they allow you to sell from their main site, through an affiliate number they give to you and you get a percentage of any sales from customers that you drive to their site.
If you want to try, you are going to have to contact each parent company individually to see if they do in fact allow internet sales, and then if you can meet their crieria, apply for a license, or sub-license.
A lot of people who you see actually selling authenic designer goods, are getting them through liquidation sales, close-outs, etc..The only problem with this method is that you don't have a constant supply. You just have to hope and get what is available, when it becomes available.
Some people, cut deals with local dept stores and arrange to buy all of their 'rack' items. Rack items are the clothing items that have not sold before the new stock has arrived and is generally put on sale. Alot of store managers would rather sell these items in one 'lot', to get rid of them than to put then on the reduced sale shelf at the store..
Keep in mind also, that unless you hold a license from the parent company, that you cannot use their logos or even their stock descriptions of their items. However if you do legally aquire the items, you can legally sell them under the 'first right of sale' law.
I used to sell a good many tommy bahama men's shirts. I tried to obtain a license from them, but not only did they require a brick and mortar store, but it had to be of a certain size in addition. They also dictate at what price you must sell the shirts for and the quanity ($5000.)you must purchase at a time.
Luckily for me, I made a deal with a local boutique to buy all or her end of season tommies and that worked well for both of us until she closed the boutique..