Approvals:
There are companies which will send you a selection of stamps by mail. You can pick the stamps
you'd like to keep, and return the rest with your payment. Prices tend to be high, due to the risks and
time involved, but this is still one of the most convenient ways to add to your collection, since you
don't have to leave your home. You usually get a month to look at the stamps.
Some approval companies will send "packet approvals", which is a selection of stamps priced at a few cents each. These can be fun to go through.
A good approval company will ask you how much you're interested in spending, how often you'd like to receive a selection, and will pay the return postage for you.
The difference between two stamps can be as little as a perforation or watermark variety. If you want stamps where no two look the same, look for packets which are "all face different". These will not include minor varieties. And some countries don't have minor varieties, so this is not an issue.
Some mixtures are "off paper", which means you'll get a bag of stamps, no soaking needed. A pound of stamps off paper can have up to 7000 stamps! Some collectors enjoy sorting through huge amounts of stamps. These are the folks who enjoy mixtures.
I am often asked how many stamps are in an ounce, so I analyzed a variety of mixes, and came up with these numbers:
Normally, you place bids by mail, and the folks with the highest bids get an invoice for the lots shortly after the closing date. When you pay the invoice, you get the stamps shipped to you. Some auctions add a 10% buyers fee, and if you buy heavy lots, the shipping costs may surprise you. But if you read the fine print, and ask fellow collectors to recommend good auction houses, this can be an excellent way to add to your collection.
I have never had a problem with any auction house. But I have some rules: I only buy at auctions outside of the U.S., where the exchange rate is favorable; I only buy wholesale lots and collections, and I keep an eye on the shipping weight.
I have to recommend Vance Auctions in Canada to anyone who's interested. They're good folks who put out an incredible variety of lots each month.