You only need to count your part of the money. In your example above, that would be the $50.
Here is your answer directly from the IRS:
Revenue for consignment stores comes in the form of commissions or fees charged from the selling price before giving the seller her share. For example, a consignment shop that charges a 30 percent commission might list an item for $100. When that item sells, the store keeps $30 and pays $70 to the seller. The store reports only the $30 on its taxes, rather than reporting $100 in revenue and $70 in inventory expense.
You do not count any money in an account for someone else unless you have a policy that it is forfeited after a certain time and becomes yours. If that is the case, it is income to you.
