Whenever you are buying something in a dream, you are 'shopping' or exploring the idea of what is valuable to you - along with the idea of self-value. If it is a type of clothing, the message can be tied to exploring your identity. Jewelry can represent the things you do to impress others, while furniture represents the ideas that you 'rest upon.' A car would be changes that are being made to your motivation.
Purchasing or buying can be a word play on ‘buying into something.’ What you get ‘in exchange for payment' can show how something valuable must be traded (sacrificed) to achieve something more necessary to your sense of well being. See Marketplace and Shopping Center under Houses and Buildings.