"It also means that for four weekends of the year, we can't use the theatre as a performance space."
Cunningham said access to the theatre, the stage and under the stage was difficult for many.
"It made sense that we find somewhere that was practical, flat and easy to load stock into.
"And there's much better car parking [at the Edgar Centre].
"It means people can fill up a box, they can pop it in their car, grab a cup of coffee and go back and do it all again.
"You could spend all day there."
She said the extra space would also allow the book sale and the Anything But Books Sale to merge again.
"It gives us the opportunity to return to selling some of those out-of-date things that people have bemoaned that we don't keep any more because we haven't had the space.
"Now we can sell things like the nice books from the 1940s, 1950s and 1960s which most people don't read; but those who do, don't have anywhere else to find them."
She said the relocation might be unpopular with some of the "traditionalists" in the city, but it would allow the theatre to raise a lot more money.
The sales are an important source of funds for its capital projects.