Buy it now

I am totally new to this and I don’t know jack about code. On my new auction site I have buyers list the quantity of the item they are selling. They also list if the price is per item or per lot. The problem is at the buyers end. Actually 2 problems. I would like a “padding” after the buyer hits the button. Like a “are you sure you want to purchase this item now?” That would cut down on mistaken purchases. The other problem is there is no way for the buyer to purchase the quantity available from the seller. If the seller has ten and the buyer wants 8 how is the transaction handled? There is an extremely good chance I am just missing something. Any help would be greatly appreciated.