Historical and biblical paintings are difficult to sell. They often involve death and many show a 
crucifixion; people shy away from scenes of martyrdom. An exception is St Sebastian, killed nakedby arrow-shot, which offers a certain homoerotic appeal. (A German gallery once tried to exploit this by selling St Sebastian pincushions in its museum shop.) …
Grand drawing rooms are a draw, as are bathrooms by Pierre Bonnard and hotel rooms by Henri Matisse (particularly if they are in Nice). Deserted Danish rooms with a single chair are hugely desirable; interiors of churches are not.
Railways are good news in paintings. People like trains. As symbols “of a mechanical progress that is now part of history,” they are both exciting and nostalgic. The impressionists were particularly keen on them; the faster the journey, the more blurry the landscape. And if railways are good, railway stations are even better. No artist beats Paul Delvaux for juxtaposing female nudes and rolling stock.