How can we sell/market-test an invention before patenting it?

If you sell or market an invention, here in the US, before filing a provisional or non-provisional patent application, you will forfeit all patent rights outside of the US, and you must file your US application within one year of selling or marketing your invention or else you will also lose your US patent rights. In other words, in the US, you have a one-year grace period - for foreign patent rights, you do not have such a grace period.