Sigh ,afcourse ballancing is needed ,logicly.First of all who says you would even be able to buy panthers? Germany would have to agree on that ,and some factors should be in place to calculate the chances of being able to buy stuff. (like being of same political ideoligy should help for instance)
Secondly the more advanced stuff should be much harder to buy ,especially when the selling country is at war ,and when that nation has a technological advantage over it's enemy's. (so wanting to protect it's advanced tech ,and keep it's own material for war)
Thirdly it should be hard to buy such stuff at mass ,especially for the smaller country's. (financial limittations ,not to much material on sale)
Fourth ,a smaller nation wouldn't usually have the doctrine's to make as good use of that material.
A human could find exploits in almost any feature if no limitations wouldn't be in place for it ,you would reason as such that a game shouldn't have feature's at all because they could be possibly exploited.
Every feature that is implemented must have limitations to make it non-exploitable ,and when it's exploted anyway (wich happens with many feature's) patches are made to adress that usually.
We have touched that part already ,exp forces are loans for allies ,this is about selling of equipment usually to country's that are at peace or not in one of the big alliances .
And it's historicly justifiable.Masses of country's bought material ,in fact few nations except the major ones actually produced their own material.Though most material was sold at peace time unless under allies ,country's at war usually didn't sell material so it should be near impossible to buy stuff from a countryat war ,perhaps unless it's really obsolete for them.
Did you know that prior to Barbarossa ,russians bought a number of PZIII's from Germany ,while Germany bough a number of tanks from Russia?Just a little side-note.