Yes Mick - there is no getting around it, the environment for CD sales makes all the difference. I have played some events where the stage is as you have described and I know there is no chance of sales happening, in any reasonable number.

I do try to set up the sales area to my benefit, when I can.

Festivals normally provide good space and often help (for which they take a piece of the sales). Concerts almost always do the same and sometime do NOT take a percentage. Pubs, you're on your own! As a solo performer, mostly - I have to place a low priority of my time to CD sales - I am somewhat at the mercy of the environment.