First and foremost I look for animals I like. If I find one I then start researching the breeder/seller. I check out the BOI. Do a general google search on them and usually ask for references that I DO follow up on.
In some cases I don't do these things because they already have a lot of good feedback. I do now however ask everyone what methods they use for shipping. The one time I didn't think to ask because it was a relatively well known breeder I got DHL shipping, *shudder* and though the snake arrived safe and sound I don't want that fright again.

I am not one for phone calls. In fact, I've talked on the phone with only three people that I've purchased from. I prefer all the communication to be in the form of emails so that both our hind ends are covered in the event of catastrophe.
If a breeder has a website I prefer to see photos of the exact animals they have for sale instead of a 'price list' or 'example' animals. And I like it to be up to date. Nothing more frustrating than inquiring on a snake only to find out it sold months before and in fact they have nothing available at this time. Grrrr
Also, if they refuse to send additional photos when you're inquiring or just say 'they are just normals or some such nonsense' I will not do business with them, why should I bother when they obviously don't care enough to put in some effort?
The size of the breeder doesn't make a lot of difference to me. I'm quite willing to try out an 'unknown' just as long as I get good vibes and their animals appear to be in great health. I won't purchase from anyone who doesn't care about their animals.