Split 50/50 buyer side open, seller side closed.
Manufacturer sells it to the seller (wholesaler) who then sells it to the buyer, who then retails it.

The commission on the sale is split 50/50 between the buyer and the seller. The sellers commission is closed (Locked in at 50% and is not adjustable, but the buyers commission can be altered and split to include other buyers) So the sellers commission will always be locked at 50% no mater what further deals are done after the sale.
The seller has a closed deal on the commission.

Fuel oil and fuel has a long chain of sellers and buyers..after it is refined.

English term (edited): buyer side open, seller side closed

Well, this is nearly a guessing. The price is fixed, so there is no room for bargaining.

By then, when a buyer wants to buy, before s/he proceeds, s/he must first pay a commission (in this case $5 per unit) to open negotiation with the seller. Please note that the negotiation is about other things outside the price.

When the deal is reached (=closed), it is the seller turns to pay the commission. The following links (esp. link 2) may be a good pointer where "open" and "closed" point to.

Link 1:
COMMISSION: Buyer pays 3 % commission, as stated below: 1.5% Buyer’s Side (Open) 1.5% Seller’s Side (Closed)

at: http://www.vita-hgta.com/ROUGH DIAMOND 400,000 CT(END SELLER...

Link 2:
If the Seller is paying any closing costs or prepaids for the Buyer or providing any funds through a financial assistance program ...., and the Lender, Buyer or Buyer’s agent attempts to have the Buyer’s side closed by anyone other than ATWOOD ... then Seller will pay no closing costs or prepaids for the Buyer or provide any funds through a financial assistance program ....

In the event SELLER agrees to a split closing, where the buyer closes some place other than ATWOOD ..., BUYER will be
responsible for any additional fees charged to SELLER incurred by the split closing.

at: http://www.avehart.com/images/pdf/Bedrock_Builders_Contract_...

For an agent or intermediary it means that the buyer is already committed to a third party who will be paid the commission for that side, so there is no opportunity to obtain a commission as a broker for the buyer. That opportunity is "closed".

See detail at http://www.giagroup.com/commission-sharing.cfm. In particular:

"There are two sides sharing the commission on a 50/50 basis, the seller’s side and the buyer’s side. In all most all cases the seller side of the commission is closed. If the seller side is open then the broker that takes the offer direct from the seller [is] entitled to the whole seller side commission. It will be up to the broker to accept any, or with whom he will share his commission. For that reason we will not get involved in any discussion with the seller’s side of the commission."