Como dizer "plantão de vendas" em inglês

Perhaps you mean "real state on-site office"?, chances are. The place brokers or real state agents would be for seeling/real state sales information.

An on-site office has been set up by the development company (name the development company and address if you wish). It could be "an on-site office has been siet up in Jacarepaguá. Etc"

Hello PPaulo, your answer is very good. It sure helps!
But how about a "plantão de vendas" that is not a physical place, but rather an intensive selling campaign for a specific period?

Set up a booth (with maquettes [small-scale building model], videos, brochures, photos of the product)or sponsor a portion of your local festivals, fairs or other events (or in a mall, in the builder premises, construction site, etc).
For cars and even real state, it can also be called a "showroom". (it strikes me as being more ‘sophisticated’ a tad.)

In a less ‘physical’ approach, for example on TV (and other platforms) I guess it could be called “selling campaign/’ (or also "advertising and selling campaign").
Usually, we see Christmas or Holidays marketing (and selling) campaigns around. But let's wait for more answers from others experts. They could improve this answer or give more insights.