Ebay can be a great source for old cast iron cookware (and just about anything else). Before you dive in it helps to know a few things.
1) What is the item really worth? Reference books are worth their cost if you'll buy two or more pieces of cast iron. One person I know lost an auction for a pan where his top bid was $135.00. I told him he was lucky he lost because the pan in question was worth much less than that.
2) Don't forget the shipping and insurance costs. Cast iron is heavy and often bulky so it can be expensive to ship. It is also brittle and may not survive the transit unless it is well packed so definitely get the package insured. If the seller does not offer insurance pass on the item no matter how great a deal you think you might get. Packaging is also critical. When I leave feedback I always comment on the packaging. Overkill is just about right when it comes to packing an item that will never be made again.
I have received items that were poorly packaged that arrived smashed. With insurance you don't lose money but a piece of history is still gone forever.
3) Don't get competitive! If some dope wants to pay too much for something let them go right ahead and do it. Have a firm price in mind when you decide to bid and do not exceed that price.
I think PT Barnum said "A fool and his money should be parted" and I'm cool with that.
4) Look at seller feedback ratings. If your item is being listed by a seller with a 100% satisfaction rating and they have sold thousands of items that seller is doing something right. Even the new listers should have good ratings and you should carefully read the buyer comments.
5) How clear are the photos? I like to see BIG photos (there are sites like Auctiva that host big photos for free) that are clear and well lit. The iron in the photos should not be oiled up and shiny. Seasoned and shiny is OK but oil can hide flaws.
If the photos are small and blurry pass on the item.
6) Grungy cast iron costs less. There are sellers who do a great job cleaning and doing an initial seasoning. This is time consuming and often involves setting up electrolysis tanks or lye baths. You should expect to pay more for a piece that has been reconditioned than one that has been collecting dust in the basement for 40 years.
If you are willing to do the work (it isn't hard and the steps required are on this site) you will save a bunch of money.
The last point I'll stress is geography plays a part in locating old cast iron. Americans living in the Midwest near Ohio and Pennsylvania (where most of the great old iron was made) probably don't need to use Ebay. Auctions and antique stores should have a good supply of nice old cast iron. Cast iron cookware is very highly regarded in the South and West so while there may be lots of iron pieces people tend to hold onto them.
And just in case anyone asks, I do not sell cast iron on Ebay.
Hope this helps.