Praise be to Allah.
According to what we understand from the question, there are four parties to this transaction:
1. The purchaser
2. Your website
3. The distributor
4. The payment processor.
Therefore we say:
It is permissible for the purchaser to pay the price via the payment processor, in return for 2% of each transaction. This is acting as a proxy in return for a fee, and there is nothing wrong with it.
It is not permissible to sell an item before taking possession of it, and moving it from the distributor’s place, because of the report narrated by an-Nasaa’i (4613), Abu Dawood (3503) and at-Tirmidhi (1232) from Hakeem ibn Hizaam, who said: I asked the Prophet (blessings and peace of Allah be upon him): O Messenger of Allah, a man may come to me wanting to buy something that I do not possess, so I sell it to him then I buy it for him from the marketplace. He said: “Do not sell that which you do not possess.” The hadith was classed as saheeh by al-Albaani in Saheeh an-Nasaa’i.
Ad-Daaraqutni and Abu Dawood (3499) narrated from Zayd ibn Thaabit that the Prophet (blessings and peace of Allah be upon him) forbade selling an item where it was bought, until the merchants move it to their own places. The hadith was classed as hasan by al-Albaani in Saheeh Abi Dawood.
Ways of resolving this problem are as follows:
1. Just promise the customer that you will buy the item and take possession of it, then sell it to him. When you have bought it and taken possession of it, then do the transaction and send it to him.
2. It is permissible to be an agent acting on behalf of the distributor, selling the item for him in return for commission, or to be an agent acting on behalf of the purchaser, buying the item for him at the usual price, and agreeing with him on a fee in return for doing that.
3. There is also a third permissible way, which is the salam transaction. This means selling to the customer an item that is clearly described and defined, in such a way that there can be no cause for dispute, which you commit to delivering to him at a specified time, on condition that you receive the money from him when drawing up the contract with him, even if that is by him depositing it in your account, because that is the same as receiving it in your hand, in terms of the ruling. It is not right for the money to remain with the payment processor.
We have explained these three options in the answer to question no. 254652.
Thus you will know that the problem is when you do not have the item in your possession before selling it to the customer.
This problem may be solved in one of the ways mentioned above, but the first way is not appropriate in your case, because you do not take possession of the item from the distributor before selling it.
Acting as a proxy in return for a fee is valid in your case, if the distributor agrees to that and regards you as his agent, in return for commission that he pays to you. In that case, it is not valid for you to deposit money in his account beforehand, because you are his agent and acting on his behalf; you are not buying from him.
But the second way you mention is valid, which is your withdrawing the purchaser’s money from the payment processor and sending it to the distributor, in which case you are acting as an agent, either for the seller, by selling and receiving the price of his item, or for the purchaser, by buying for him and paying on his behalf. But this is on condition that there should be an agreement between you and the other party who will appoint you as his agent, and that you should state your fee for acting as his agent.
As for the salam transaction, it is not valid in your case, because the condition for it being permissible is that you should take the price in full at the time of drawing up the contract, and the money should not remain with the payment processor.
In conclusion: so long as you do not have possession of the item and you cannot take possession of it before selling it, or obtain its price in full from the purchaser, then you can only act as an agent for the distributor.
What you have mentioned about the customer paying instalments through the payment processor is not permissible, and it comes under the heading of riba as you mentioned, because if the one who provides the payment pays the price on behalf of the purchaser, then asks the purchaser to pay him more, then that is riba.
What is permissible is for the customer to give the money to him (the agent), so that he can send it to the seller, in return for commission; in that case he will be acting as an agent or proxy in return for a fee, as mentioned above.
See also the answer to question no. 102744.
And Allah knows best.