If I may add a few points to the very good ones above:
1.) Know what you are buying. Meaning if you buy either a bar or a single medal, know what it is. This may sound basic but there are people who buy things just because they 'look good'. For example, some research would show that the first medal on the bar in post 156 has extremely low issue numbers, and is heavily faked.
2.) In my opinion, bars are easier because of combination. If a bar's combination does not make sense, that is already a clue to keep on looking. Once again, lets take the bar above as an example:
a.) The first medal was given to NCO's and enlisted, whilst the fourth medal is a 24 year long service award to officers.
b.) In 1905 Bavaria gave out a jubilee medal, it also did this in 1911 to those who did not get that of 1905 (therefore you will never see them on a bar together). Therefore, it is impossible for a Bavarian bar to have a 24 year service medal and not have one of these medals.
c.) The first award is a peacetime one, which would never be mounted in front of the wartime iron cross.
3.) When in doubt, ask!
Regards,
Matthew