tantrums

OK I'm just a bit frazzled from a four year old's tantrum. Need to calm down and stop feeling so guilty. We were on our way down to the group, passing through the fleamarket this afternoon. We were on the last minute as it had taken ages to drag Fergus away from the computer and drag Freddie down the hill. Freddie had his purse, which had just over a pound in it. He saw a pot of Duplo at a stall: looked like one of those car-boot clearout kind of stalls: everything should've been cheap. The Duplo was £4. I started to think about how much I was prepared to pay, not £4, maybe 2 or 3. Then he started whingeing, and shouting, and it turned into a tantrum. Then I decided I wasn't buying it at all. I might have paid the £4 if this hadn't happened and we had had more time. But he was putting me under unreasonable pressure. So I had to put up with tantrum for the next 1/2 hour. Then on the way back we had to go through the market again. I had suggested that she might take less money for it a bit later on. It was about 2.30 by then. I offered her £2 which she refused, and the tantrum started again, leaving no room for negotiation. So we had more tantrum and a very slow walk home.
I looked on Ebay and the equivalent was the same price, plus postage. But you can't charge Ebay prices at an outdoor windswept fleamarket...? But the problem is the tantrum. I would've been prepared to go back and buy the sodding thing if it wasn't backing down over a tantrum. Now I feel all guilty and sad that he didn't get to buy anything.