It usually happens when you load the screen while someone else buys or fills an order. Just click on next one and buy that
i had something simalar where it said it had 1 or so things on market but i bought 3 or so and it stayed
This is a regular thing and has to do with CCP’s poor database handling. Especially in the night and mornings, sales from the other day show as 0 and then load hours later. I pointed this out a while ago.