The phrase "could be bought" is correct and usable in written English. You can use this phrase when you are talking about something that has the potential to be bought, whether or not it actually is. For example: "This rare book could be bought for a high price at the antique store."
Corporate bonds could be bought.
I knew blood could be bought.
The President suggested that groceries could be bought on credit.
The cheap ones could be bought for beer.
Before, only 10percentt could be bought abroad.
Saint Laurent's clothes were proof that class could be bought.
At Lauren's show, everything seen could be bought immediately.
Additional water, when needed, could be bought from farmers.
She said those gifts could be bought for about $100.
A poilu could be bought, again, on a mail order catalog.
Shoppers paid cash for beef but cattle could be bought on credit.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK