He was asked:
Two merchants were offered a commodity for sale, and each wished to buy it. One said to the other, “I will buy it jointly for you and me.” His intention was to avoid being outbid by the other and having the other acquire it alone; this was why he sought the partnership. One of them then purchased it, paying its price equally from their funds. Is this sale valid under these circumstances, or does it involve deceiving the seller?
He replied:
Praise be to Allah.
If there are others in the market bidding against the two of them, but one refrains specifically from bidding against his companion because they are buying jointly, this is not forbidden. Bidding remains open; one has merely refrained from bidding against the other.
This differs from a situation in which the market traders agree not to compete in bidding for goods they need, so that the owner will sell them below their value and they can divide them among themselves. This may harm the owner more than intercepting his goods before they reach the market when he sells them at a negotiated price. Such conduct plainly involves depriving people of their due. Allah knows best.