There are multiple problems with this idea.
The first and most obvious problem is that the sealed container which has a perfect vacuum inside (seller claims so at least) is not of much use until you can put something inside (experiment, material, ...). And when you do that, you unavoidably get some gas inside with it, which ruins your perfect vacuum.
You could avoid that problem by putting things into the box first and then open it in space. But there is usually hardly any advantage compared to running the whole experiment in space and only bringing back the results (which will often be just immaterial data which you do not need to land softly).
But let's stick to the idea for a while. Let's connect some airlock to the perfect vacuum chamber to put things inside later. This can work... sort of. The problem is that a better vacuum usually means that the pressure is lower by at least an order of magnitude than what we can create on Earth. If your vacuum is not at least an order of magnitude better than the one we have, then usually nobody would care. So, if you can pump down the airlock to your laboratory's ultimate vacuum pressure, and you want to gain at least one additional order of magnitude, the perfect vacuum container needs to have at least ten times the volume of the airlock. And you can only use it that way once. That's not very practical.
Anyway, the real show-stopper is the fact why we can not create a perfect vacuum in laboratories on earth. It turns out that the problem does not lay in the vacuum pumps. The problem is that the container itself ruins the vacuum. You can build an almost perfect vacuum pump, quite similar in effect to connecting an infinitely large container with a perfect vacuum to your experimental chamber. You just need to cool down the chamber wall to a really low temperature (think liquid helium or below). Then any particle hitting this wall just sticks there -- effectively equivalent to the situation when it would continue into an infinitely large empty container.
But such a perfect pump would still not create a perfect vacuum. Every piece of material you use to build your chamber, experiment etc. is continuously outgassing lots of gas ("lots" in terms of an ultra high vacuum where a few thousand particles per cm³ is considered a lot). So the resulting pressure is kind of an equilibrium between outgassing flow and pumping flow. In order to get a better vacuum, you usually need to pump faster, not "better". (And use containers and materials releasing less gas, of course.) And you need to pump out lots of gas (again, "lots" in the terms of an ultra high vacuum, but still a lot compared to the amount of remaining gas present in one moment inside the evacuated chamber), especially for a sustained vacuum. And a container with a perfect vacuum is not a good vacuum pump in these terms.