I’m not sure if I am missing a prefix similar to using “-” for selling. You can select the box and type a positive value however pressing enter does not confirm the value. Buying in trade center seems to be only possible via the slider currently.
I can confirm that I have tried this with goods being only sold by the trader, goods the trader both buys and sells normally, and goods sold to the trader then attempting to repurchase.
Also appears to be across all goods not limited to specific items, and I tested using values that total cost would have been allowable by the stocked gold.