Raising money by a stall at fairs, fetes, boot sales, etc.
There are no real problems with this idea. Make sure that you select appropriate items for the type of event you are going to attend. I once attended a table sale where everyone was looking for really cheap items and someone next to me was selling items at £20 plus. There was a lot of interest in what was being sold but no one was going to buy anything at those prices! So an important lesson is know your market and sell items suitable to that market at the price that people will pay!
If it is an outdoor event make sure that you can protect your stall if it rains.
Make sure that the way your stuff is laid out will not make it easy for people to nick stuff! Especially your moneybox and personal stuff.
Have enough small change available, especially if you have a lot of items priced at under 50p.
Make sure that there are at least two people always present at each stall. Security is an issue in many venues. You can lose products or personal items (even the cash box) if there is only one person working and they get distracted or are too focused on one potential customer.