prestbury this problem was aired on p1 of this thread.
here is the explanationI gave then - and is still true
The tv producers want a format that is difficult, competitive and has a defined end. This is why they do it the way they do.
We used to do a bit of antique dealing and, yes, we bought at auction and sold at fairs but when buying at auction, you might well get a competitor bidding on 20 lots losing on 17, or even not buying anything at all because they decide the items are becoming too expensive and then when the items they buy are put up for sale, they make take months to sell, if they sell at all. That is not a very good format for a tv programme, too undefined with no action at the start and petering out at the end.
By getting people to buy at a fair or visit a shop, then there is a good start with activity and things to catch the viewers interest. By having them then sell at auction, the whole things comes to a distinct end with a run of tension to see what things sell for.
People forget that the purpose of these programmes is to make tv that attracts people's attention and holds it, not to do things in a way they are done in real life.