Ok, so I'm not an expert in fashion but I know finance.

Here is my take:
These two would be considered "fast fashion" retailers or better yet, e-tailers. Fast fashion simply means that unlike Coach or American Eagle or Levis, these FF retailers don't have to try to predict fashion 6 months in advance risking a big flop and having to heavily discount items that don't sell. FF retailers simply 'scout' runway shows and buy wholesale from these designer labels. A lot of designers would like this because they are essentially getting a guaranteed sale plus added exposure. Another thing is that these FF retailers don't keep their inventory for months at a time, they do so in cycles of weeks. If a designer sells out, chances are they retailer will continue to come back for more designs from them.
They are purchased wholesale, on cash basis account, payable on credit of 30 days or 90 days.
The economics as you state it are a bit more complex that what I care to explain here, but essentially if you were to 'replicate & improve' what I would do is scout and offer purchase orders to designers, just like they do. First order completed as 50/50 paid in full/credit term payable 60 days or so (assuming you already have a store ready to move inventory and not waste those 60 days setting up). Aim to sell all inventory before 60 days and pay balance with revenues. Extend credit term to 90 days at increased inventory, aim to sell by 60 days and keep that cycle going. What this will allow you to do is to always have inventory being paid for by customers before they are due for you essentially having the clients pay for your expansion in inventory. The break even is simple, don't sell for less than what your wholesale amount is.
Typical increase from wholesale commodity goods is 30%, try that margin. If you have to discount "heavily" at 15% or 25% you still get at least 5% safe margin