Swan goose is just another name for a white chinese goose. Geese don't usually have crests but have 'tuffs'. Like
in Tuffted Roman Geese. I suspect what is offered is the offspriing of the crossing of the two above. find pictures of the white chinese and the Tuffted Roman and you may have some idea of what they will look like. BTW that is a very good price for any kind of geese.
Correct NOrman,"Swan Goose (Anser cygnoides) is a rare large goose with a natural breeding range in inland Mongolia, northernmost China, and southeastern Russia"-Wikipedia. However they are not white. What do you think the chances are that these rare geese would be offered for sale at 3 for $20?
Both the Chinese and the African geese are believed to have been developed and bred from the Swan Goose. White Chinese are often refered to as swan geese because of their swan like necks.