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Some of the target phrases are shared by the 2 campaigns because people might search for the following phrases whether they are buying or selling. It works for both scenarios:
Widget for sale
So how can I get my buying ad AND my selling ad to appear when the above phrase searched for?
Or failing that, what is the workaround?
Say it in the ad - "we buy and sell widgets."
keep using 2 campaigns and better differentiate your words. Use negative keywords to help differentiate.
'i want to buy a widget'
The problem there being that 'buying' and 'selling' as operative words are *relative*. Someone wanting to buy a widget is equally likely to search 'where do i buy a widget' as they are to search 'who sells widgets', so this is tricky.
I think I'd opt for option 1 above.
If you make one campaign devoted to buying widgets. Use a lot of negative keywords to make this campaign keywords very specific to buying widgets. This campaign should have a landing page devoted to buying widgets.
Do this again for selling widgets.
Then make a 3rd campaign for the general widget inquiries where you want to show up, but you can't tell by the searched terms if they are buying or selling widgets, with an ad 'We buy & sell widgets' like above - then make a few custom landing pages to test out the conversions. This campaign could have a landing page with quick links to different aspects of your site for both buying and selling.
Then make a few other custom landing pages with different versions of the above as you want them to click from that page to their specific category ASAP so they don't leave your site before moving to the page you really want them on. Test out the conversions of these landing pages to see which one works best for you.