Honestly, it is less of a problem than you think. Most people will remember not to pour your water in the tank.
The only way you can guarentee the fish not to transfer disease is a 45 day QT at 80-81 degrees or so at 2.5ppm copper power (or similar) with no new additions. Jay Hemdal has a guide. The risk you take of doing less than this is that you could contaminate the non qt tank and then have to keep it fallow for 45 days.
I see, definitely will think more about this and check out @Jay Hemdal guide.
Most of the stores I know that run copper, since their fish are going straight into the display tanks, they are sold within days.