Every listing has two buttons. Add to Cart keeps the item while you carry on browsing. Buy Now takes you straight to checkout with that one item.
Why your cart is split by seller
Items on eBuy come from individual collectors, not from one warehouse. Each seller packs and posts their own parcel, so checkout groups your cart by seller.
Grouping helps you see who will fulfil each item. It does not combine shipping charges: every available listing keeps the shipping amount shown on that listing, including when several items come from the same seller.
What you pay
The checkout total is the price of each available item plus each listing's shipping charge. The final total is shown before payment.
If your payment fails
Nothing is charged for a failed payment. The items are held for a few minutes before you can try again, so that a half-finished payment cannot sell the same item twice. Wait for the time shown on screen, then retry.