Ideally you don't want to spend loads of money doing up the house you're moving from.
Buy plants that can be put into pots and then you can take them with you. Don't include potted outdoor plants automatically. If the buyers want them, let them offer a few quid for them
Suggested plants include confiers (Dwarf ones) and winter bedding plants. These are very cheap and you get some pretty good colours too - look at these pansys from thompson and morgan.
They're ideal for flower beds or troughs outside your front door.
Very welcoming in winter, but not perhaops suitable for a swanky city centre flat where a palm might be more suited.
Keep them watered, fed and weeded too. Neglected plants look worse than no plants.