I wrote a long post a long time ago chock full of tips on vintage shopping, which you can check out here if you are so inclined.
I have something to add, especially for online and long distance shopping. Ready?
ALWAYS CHECK YOUR MEASUREMENTS.
It seems obvious, and yet sellers make it real hard. Have you noticed how often craigslist sellers fail to include measurements in their descriptions? Vintage sellers, you know what would be an awesome added bonus (in addition to listing measurements)? Photographing your items in context of other items that can give a sense of scale.
I recently found this amazing set of 8 vintage '70s Danish dining chairs for a client.
Love at first sight.
But guess what? No measurements. And when we got them from the seller, it turned out these gorgeous chairs are practically child-sized. If they had been photographed with a dining table, we would have known from the get-go that they are barely the height of a table. And the seats are only 2 inches wider than a set of child chairs I have from Ikea.
I'm curious: have you ever purchased something from photos and been surprised by the scale when it arrives? It can happen!