Having purchased a few anvils let me impart some advice. I bought my first one off Craigslist and drove to Paris Texas for it.
It is a 235lb peter Wright. I gave $600 for it and that was a steal at the time. It is still a good deal by today’s prices.
Look for a mouse hole, peter Wright, solders, etc.
Examine the top(face) careful. If you can see it in person, take a regular hammer with you. Hold it above the face with the handle and head parallel to the face about 5 inches above. Pivoting at your elbow let the head strike the anvil. Let gravity work here. It should have some bounce.
Depending on what you are willing to accept as far as imperfections. This will influence price. If it has torch holes in the face or the sides of the face are gone or heavily cracked and falling away this will drive the cost way down. Wxpect to fins some wear on the face edges… If you see cut or chisel marks in the feet this will tell you that someone actually made cutting tools with this anvil. It was common to demonstrate edge retention on the feet of anvils.
Beware of painted anvils. If it looks to good to be true it likely is. Sometimes people dress up an anvil with bonds and paint.
Do not be afraid of surface rust on anvils. Even a little pitting is OK if you have the ability to refinish it.
Tons of resources on buying an anvil can be found on the web as well.