Comparing/combining 2 columns into one

What I'm looking at doing is comparing and combining (or maybe more mixing) 2 columns onto 1.

Columns A and B could have values of say Buy, Sell, N/A and blank. There could be occasions where a row has the same value for A and B, or otherwise one column would be Buy or Sell, and the other N/A or blank.

I would have a third column that compares columns A and B, and reports Buy or Sell if both columns the same. If one column is Buy or Sell, and the other is N/A or blank, the third column would report Buy or Sell.

I think it's this last bit that complicates things, as there is some sort of logic check here.