To select all buildings that fall into a selected circular buffer with a 2000 radius you need this command:
SELECT * FROM Buildings WHERE ST_DWithin(geom, (SELECT ST_GeomFromText('POINT(0 0)', 26918)), 2000);
See this, for more examples.
I've gave you already the right answer!
To prove you I'll show the tests I've done before answering to you, on my test data (my SRID is 31700):
If you want to see the selected points in QGIS then you need to follow the images bellow:
So, I think that your center point coordinates are not [0, 0] and if you'll follow carefully my demonstration you'll obtain the proper results!
My tests showed me that it is possible a user to keep its data in a PostGIS table with the 4326 SRID, and to obtain a proper result, like above, using its own SRID (e.g. 26918).
In this case one my use this command:
SELECT * FROM Buildings WHERE ST_DWithin(ST_Transform(geom, 26918), (SELECT ST_GeomFromText('POINT(0 0)', 26918)), 2000);
Yet, in this case, I've noticed that the selected points are still in the 4326 SRID. Therefore, if you create a table based on this query, eventually, you need to update the table SRID to your own.
Here's what I've done (using my test data) prior to see the selected point in QGIS:
But, the newly created table has the 4326 SRID:
Therefore, we need to change it to our own:
(I've verified my solution, considering the buildings are points, polygons and multipolygons, and didn't noticed any other problems than those already presented here.)