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If you inherited the business - your basis is a fair market value of the business at the time the decedent died. That is so-called stepped-up basis.
If I understood you correctly the FMV was determined back in 2005 as $150,000
The basis reported is most likely a book basis that the company is required to keep track of.
You will report the sale of your share on the schedule D and should use $150,000 - that will give you $20,000 capital loss.
I suggest to attach a note to your return with information to the IRS that the share was inherited and how did you determine the share basis.
Your capital losses will offset any other capital gains, but if you have net capital loss - your deduction against your other income is limited by $3000 in the current year with the rest to be carried over to the next year.