I think the first post and the fact that there is a small pool of people who appreciate high quality vintage is correct.
Most throwaway current clothing is probably going to be rags in a few years time, and much of the stuff that I see in charity shops in London is of this quality. But there are a few items that can suprise (and it's always worth looking). I have many quality ties, including Gives and Hawkes, for circa £1 in charity shops, and a three piece Daks suit in excellent condition (with braces!) for £35!
But most of my clothing come from vintage shops or eaby, as people with Savile Row suits to dispose of now realise that there is some resale value in them, as opposed to the often negligible value of the things that get donated to the charity shops.
However, as has been mentioned, the market for high quality vintage is small and the prices not that high considering the quality.
For one, much vintage clothing sold here in London is bought by trendy youngsters who want a look but don't want to spend much money (or have much money), but they don't wear it 'seriously', as some of us do (they'd maybe wear a tailcoat with skinny jeans as a look for example). This accounts for some of the less good quality items on sale at the majority of vintage shops.
Then there is the small number of people who wear vintage seriously from time to time, or always wear vintage, like myself. I am currently wearing a beautiful 1967 Meyer and Mortimer grey chalkstripe three piece suit in very heavy material, working cuff buttons etc, it is in exceptional condition and will last for years I'd wager (and is very comfortable by the way). It cost me less than a new suit in Marks and Spencers would, and they are simply not comparable.
People who wear these kind of things in a way are eccentric, certainly the majority would not want to eschew their jeans or 'modern' suits no matter how good the quality of replacement, and paying e.g £100 for dress up is probably beyond most people.
Vintage is a statment as well as a term for old clothing. I simply love the look of classic Savile Row tailoring, and that English look. I don't have the money even for MTM suits, but I do for vintage finds and a good alterations tailor. It is simply my wardrobe and my passion now.
Some people also seem sniffy about used clothes, which is not a proiblem for me. And as eluded to earlier, if you have the money for bespoke quality garments, you're probably going to go bespoke.
I am always pleased and almost astonished to find superb vintage clothes for silly prices, like my Savile Row tweed suit from 1984. Exceptional quality and condition, and less than a tweed jacket would cost you from a department store now.
P.S: As for pitfalls, fit and condition only really come into play with ebay (and fit is also an issue in new clothes via ebay), in vintage shops you can assess that yourself and buy accordingly.