Worse service ever

I went to the store to buy 25KG of 3inch nails and 50KG of 4inch nails and when i got to the store there was only one packet of 25KG 4inch nails left and i took it and as i was about to pay i was approached by a lady that works there and said to me i cannot pay those nails because someone already paid them. When i asked how is possible that someone paid for the nails yet they are still on the shelf and they couldn't give me an answer, i them asked to speak to the manager and he was so rude. they couldn't show me the receipt of the purchased one but all they can show me was a quotation. i left the store very very disappointed