There are various modern meanings of the word 'stall' but most if not all can be traced back to the idea of standing. A market stall is a place where a trader stands to sell his goods, a choir stall is a place where a cleric stands to sing. A horse or cow stands in its stall and so on.
A staller was someone who did his job by standing in a set place. He was stationed in his position. Station is word which derives from the Latin word coming from the same ultimate root as stall, the verb sto, I stand. A staller then, was someone who stood. He might be a collector of customs dues at the gate of a city but I believe Eadnoth had a more specific job. As France had its Constable so pre-conquest England had its Staller. In modern terms, he was chief of police or head of security. You may have noted the stable part of Constable. That too, evokes the notion of a fixed post. Perhaps it is not too fanciful to picture Eadnoth as being notionally, the man who stood behind the king to see that others did not stab him in the back.
Another specific use of the word staller, or more usually stallar is particularly Scottish and much later. A prominent churchman might pay a deputy to stand in his place when attendance at cathedral services was required, in the same way as an absentee parson might pay a cleric to do his work in the parish.