Hi Wombat
Sorry didn't see your post ... CFDs may not necessarily affect the market unless the underlying provider hedges their position somehow. Market makers for example, take the other side of your trade - so you will never have any issues trading CFDs with them ... apart from the fact that they will try to fleece your money
The CFD providers that do hedge may refuse to offer CFDs on a company if they can't hedge; for example when short selling was banned, IG did not offer any short selling.
Cheers
Deon