We periodically receive updated price lists with about 5000 items. I have two problems that I need help with.

The first problem is that some of the part ids are in the format XX5-0808, some have an additional letter XX5-1206K and some have two hyphens XX5-06-04K. There are about 30 different combinations of numbers in the part ids. In order to import the items into our system I need all the part ids to be in the format with the two hyphens. I need a worksheet function that does the following:
if two "-" in the cell then do nothing
if one "-" in the cell then insert "-" 2 characters from end, unless the last character is a letter then insert 3 characters from the end

The second problem I have is that while the updated price list contains 5000 items we hold only about 1500 of those items. I can export our item list to an excel file. I need to find a way to compare each item and mark the items where the part id appears in both worksheets (or mark the items where the part id appears in only one worksheet) so that I can import only those items that we stock.

You need to write some code, or to hire somebody to do it for you. Depending on part on your version of Excel, and in part on your skill, motivation, and willingness to write code, your options are Visual Basic for Applications, and .NET for Microsoft Office.

http://www.functionx.com/vbaexcel/index.htm
http://www.ozgrid.com/VBA/

You'll need looping constructs (for each item in your list), you'll need to get used to IF THEN ELSE logic, and string manipulation.