Almost every single vintage dealer started off small just by selling off items from their own collections to fund their habits and then eventually realized that it was time to make it a "real" business!
I started off that way too, selling off items from my Victorian and Edwardian costume collection so I could specialize in corsets and riding habits but in the process of parting with things, started to see what got better bids than other things. For instance with me, I found that my 1860's garments (even if they were in poor condition) sold for far more than a mint condition 1910's dress so this is why today, I tend to gravitate towards earlier costume as 1) it's my favorite period and 2) I now have a specialized customer base for these items that I have built up over the years.
I don't want to be negative, but I've seen vintage businessess come and go- there have been a few around me that this happened to. People have an interest in vintage, buy up all the vintage they see, get business loans, etc and then close down as they never got their feet wet by going through the initial and VERY important learning process of testing out the market, etc by selling items on a low scale basis from their own collections.
I learned SO MUCH from this including making mistakes (don't even bothere buying shattered items so matter how beautiful!) with very little risk (things in your collection tend to have already been paid for so you don't have to worry about paying off loans, etc for stock).
I'm not saying that you don't already have experience in this but I have to agree with the others about getting stuck in the sooner the better by selling off some of your own things to test the market and then charting your sales as there really isn't any standard statistics for vintage clothing (which is pretty much true for any aspect of the antique trade!).
Lei