The marketplace, which has already signed up sellers offering 2.4m product lines since its launch at the beginning of November, has launched "Onbuy Basic Seller" in response to feedback from customers.
OnBuy basic sellers pay no subscription fee at all, and OnBuy charges a selling fee of just 6% on product sold – far cheaper than rivals such as Amazon.
OnBuy is continuing to offer its unique no commission £49-a-month subscription for the majority of sellers, as well as its premium no commission £89-a-month subscription for managed sellers.
Cas Paton, managing director of OnBuy, said: "Our commission-free model is working brilliantly for many retailers who are benefiting from the fact they don't have to pay selling fees.
"We have listened carefully to feedback from some of the smaller sellers who don't want to a monthly fee that may absorb a large portion of their profits.
"We have decided to add an extra tier to help support these smaller sellers so they can get their products online quickly without a monthly commitment.
"While they don't have all the benefits that come with the subscription offer, such as API access, advertising and account management, they will still be paying the lowest commission fees online among our competitors.
"OnBuy's vision has always been to work with sellers, not against or in competition with them, and we feel this additional offer will help support the smaller retailers.
"We want to leave it to sellers to make their own minds up about what works best for their business."
Larger sellers are already benefiting from OnBuy's unique commission-free model in which they pay no selling fees and pay a simple, straightforward monthly subscription.
Sellers were first invited to sign up at www.OnBuy.com/sell on November 7 to register for OnBuy, which is on track to get thousands of sellers and millions of products on board before launching a consumer campaign next year.
Confirmed sellers include World of Books Ltd, the fourth largest Amazon seller in Europe and the largest Amazon bookseller in the UK
Sellers signing up to be an "OnBuy Seller" and paying the £49-a-month subscription pay no commission fees, as well as the extra benefit of API access (if required).
Sellers can also upgrade to become an "OnBuy Managed Seller" and pay the £89-a-month subscription, which includes no commission fees, API access, advertising opportunities, prominent placement and tailored account management and support from the OnBuy team.
The subscription model means that for the vast majority of sellers, the cost-per-sale is lower than those charged by Amazon and eBay, allowing vendors to sell at lower prices and pass savings on to customers.
OnBuy is growing every week and is expected to grow dramatically in the next three months as products in dozens of categories are uploaded, including electronics, toys, clothing, jewellery and furniture.
OnBuy is a trading platform and not a retailer – it does not hold any inventory so unlike other major online retailers, it will never offer products in competition with its own sellers.
Payments are securely handled by Stripe, giving sellers confidence and peace of mind.
Navigating through the OnBuy site is designed to be quick and easy for shoppers as they are clearly directed to the best prices for a huge range of products.
Cas said: "Our research tells us that customers and sellers don't like some aspects of Amazon but so far they feel there has not been a credible alternative. Now there is one.
"The question for sellers shouldn't be 'why would I sell through OnBuy?' It's 'Why wouldn't I?'
"There are key benefits to signing up now and not waiting, including getting early exposure and gaining market share. Sellers will be building a relationship with OnBuy and will be directly involved in the huge marketing push.
"The benefits are not just for sellers and customers, but for the economy as a whole. As a British company, OnBuy pays its taxes in the UK and is passionate about supporting the British economy.
"We're very excited to be at this crucial stage in the development of a marketplace which we believe will shake up online shopping and offer both sellers and shoppers the deals they deserve."