It appears to depend on the initial quantity that the player brings into the dockscreen.
E.g. corporate enclave and titanium ore:
Start with 20 tons, sell them, get another 20 tons -> Corporate enclave not interested in buying any more.
Start with 50 tons, sell them one at a time -> Corporate enclave buys up to 25 tons, price for each dropping as inventory increases. Alternatively, sell them all at once to bypass sell limit and receive full price.
There are several bugs and balance issues with this phenomena, i.e. the gameplay effect this has on ships with limited cargospace.