A few random thoughts and a list of best pickings!
Nearly new sales are second hand sales with pretentions. The goods are just the same.
A useful piece of advice that was given to me, is to go for the most expensive or rare item on your hopeful list first.
It goes without saying that it is best to be at the front of the queue-this is an admonition to myself-not good at getting to places early on Saturday mornings. Don't think that because a sale is in an expensive area or at a private school no one else will want to attend-this isn't true. It is amazing who you meet at second hand sales!
Don't buy trash-just because it is second hand doesn't mean something should be frayed or stained.Check zips.
It isn't worth buying more than is needed although it might be worth buying quality children's clothing in a larger size.
Some places only accept cash and in fairly small denominations-up to £5 notes are usually acceptable. I've not had any problems with having £10 accepted when paying over £10.
Just because something is sold second hand doesn't mean it is cheaper than something new. I've seem books and strollers sold for more than the new price.
-baby sleeping bags-check the zip first!
-children's clothes especially coats and dresses
What are your second hand sale tips?