I'm just reviewing the rules and I saw a strange omission.
The rules state that initially the 4 ancient wonders should be put in the market, making them available to build.
Also it is stated that if one of the players are the Egyptians they should get 1 random wonder and that the now vacant place in the market should be filled with 1 random medieval wonder.
However nowhere in the rules (especially not on page 17 which details the building of a wonder) is noted what should be done to fill the empty place after a wonder has been built.
Now I guess the correct rule should be to draw a new wonder from the wonder pile and put it in the vacant space so there are always 4 wonders available to buy (until they really run out of course).
However this is not written in the rules and perhaps other ways were thought of to change the selection of the wonder market?
For instance, can you obsolete wonders that have not been built yet?
(personally I would say yes; this way a player who concentrates on wonders can make new wonders available even if nobody else builds them)