Don't forget Cat, dealers prices and ebay prices are not comparable. I think this has probably been discussed before in the cafe. Dealers have higher overheads, they often have to wait a long long time for a piece to be sold. Ebay is either top dollar for rarities, or a haven for bargain seekers.
And don't forget, unless you are very sure of your dealer, you have the risk of the piece arriving in good condition or not, so a lot of people still don't like to buy breakables on line, and would prefer to pay the premium price for being able to pick up and examine a piece before they buy.
it's like Apples and Oranges. Both delicious fruit, but not from the same tree..
In general, whilst I like to get near the online market value, as long as I made a reasonable profit I try not to be greedy.
Which is not to say that I don;t grind my teeth when I see Miller's valuations.....
