remove white spacing between posts

You should create a Child theme to make your changes. If you do not, all of your changes will be lost if you update your theme.

Try adding this to the end of your child theme’s style.css:

.entry-content > p > iframe { height: 150px; }

Most of that space is being created by the embedded iframes — which have an inline height of 200px. You might be able to override that by setting a negative top margin on the “Posted in Marketplace” div

I don’t see the code at the end of twentyeleven/style.css.

Now you are missing a closing brace that was just before that line:

}
.entry-content > p > iframe { height: 150px; }

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