It really depends on the extensions, not all one letter names have value if they are in crap extensions. Not all 1 letter names are available to buy/register either.
.AD (Andorra) has no sales history at all so I think that would be a waste of time.
.AE (UAE) would certainly have value if you could get one but I don't believe they are available to the public
You are better off focusing on .com names when new to the industry, get some sales under your belt first.