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When the business reports the receipts and expenses for tax purposes on the tax return, the IRS will then have the information as to what is, or is not, profit and included as subject to income tax on the tax return.
It very likely is best to reflect in the company books the gross amount received from the buyers as receipts and then to show the payments out to sellers either as an expense or as cost of goods sold. Of course, documentation of the payments out must be kept and retained as proof payments were made.
If there will be Form 1099-K sent to the IRS by third parties (such as PayPal, or others) showing the total payments in from buyers and the amount paid to buyers less your profit out will allow the IRS computers not to send an automatic notice for the amount of the 1099-K being reported to the IRS and not apparent as receipts on your tax return.
Although the amount reported by others to the IRS is seen as being received by your company that does not mean that it is seen as profit. It is only seen as received and not routinely presumed to be retained as profits.
Only in the case when the taxpayer does not file a tax return does the IRS use the amount reported by third parties to construct a tax return in lieu of the return that was not filed. In that case the reported amount might be (mis)construed as profit but that only happens when a taxpayer fails to file and the IRS does not have the reported payments out to properly reduce the receipts to the actual profit.
Please ask if you need clarification or more discussion.
More specifically to the last part of your question as to what forms are needed, please understand that it is not the terms of the agreement between you and the sellers that will dictate what is taxable as profit; but what actually occurs in your business.
A fundamental of taxation is that substance rules over form. That means it is not so much how you and I try to name or characterize a transaction that determines what is taxable; but rather what actually transpires between you and I that is what counts.
Obviously attorney assistance in the contracts with sellers will be important to protect the seller's right to the buyer's payment less your fees and the buyer's and seller's means for settlement in any dispute; but does not dictate what the facts of the transactions will be for inclusion as income subject to tax.
Hope that clarifies the last item I had not directly addressed.