The market won't change too much. Everyone can afford food and shelter and clothes now, in the former 1st world countries. There are places in the world where people can't get food and thus die in the streets, but this is a rounding error here. Therefore not much will change at the bottom end. Prices might drop slightly due to reduced demand because people will occasionally splurge.

Note that this won't actually help with many social problems. For example kids who go to school hungry because mom spent todays money on crack will after UBI merely go to school hungry because mom spent todays money plus UBI on crack. Neither the slumlord nor the supermarket will see a penny more. Cultural changes are much harder to make than mere economic changes.