I mainly send and buy Taobao cards in boxes.
Because indeed some boxes have copyright issues but not most, especially cute ones.
Those cards selling as stationeries are mostly massively produced and designed by local stationery companies, such as 拾光，信的恋人 etc. So they are also sold overseas.
Most cute postcards are from stationery companies I mentioned which also produce original design washi taped, pens as well, very hot in China. And their products are also sold in tourist attraction in China, I have seen several stationery shops near hottest tourist attractions.
I guess some are very sensitive due to those cards that have copyright issues that they are against all cards selling on Taobao. Not all!