You see, I'm an avid eBayer. I've used eBay since I was around 17 and not even old enough for a debit card. I buy and sell (not a business seller though) and have recently been on a bit of a vintage eBay adventure. I currently have around 30 items in my watch list that are from the 40's and 50's and there are some real diamonds in the rough. I shan't be buying most of them, but it's very handy to watch them and see what they go for in terms of cost.
With eBay, you're either standing on a goldmine, or in a sinking sandpit.You have to be a discerning shopper. There's no point in using eBay if you don't know the ins and outs of the language on the site, the rules and more importantly, your subject matter. Now, I've been into the vintage fashion world for little over 6 months and already have a bedroom full of true vintage items. Some expensive, but most bought from eBay or vintage fairs.
However, the reason I have so much vintage clothing is because I'm savvy - I buy clothing based on my measurements only. I have a 26" waist and a small bust, so I know that a coat with a 40" bust will swamp me, no matter how pretty it is. I supposed you have to be rather cut-throat. If something isn't to your measurements (too small or too big) then click the back button and forget about it.
Buying vintage is a skill, unless you're one of those traders who bulk buys bags of crap from a wholesale warehouse.(Yes, I'm a snob!) It takes time to sift through the rubbish and find the real gems. Buying things with holes or stains is a real risk. Rust is one thing that does NOT wash out of clothing so if something's got rust stains on, walk away, no matter how much you love it. You won't regret it, I promise. (Rust stains will be orange in colour and will most likely have seeped through to the other side of the fabric).
Again, holes in netting are very difficult to camouflage, as are holes in silk or satin. So avoid.
I tend to use the phrase 'true vintage' as on eBay you'll find many a seller who finds it necessary to sell brand new (or second hand) modern clothing, but describe it as vintage in their listing. It's not until you get down to the detail you find out it was bought from Primark. Again, avoid any seller who does this knowingly. It really gets my goat.
Sadly there are people out there who will bid on something without realising it's not genuine - I once saw a lovely looking tea dress described as 'WWII vintage tea dress' and it turned out to be a dress from Next. It created somewhat of a bidding war - probably due to the fact that the -albeit clever- description from the seller saying how rare it is. How rare it really was, I care not to ask!
So that's just a few bits of advice I can give to people, things I've learned along the way.