What is it about Op shops?

Judi from Kamo

What is it about Op shops that makes people so bold?
I love browsing in Op shops and if I'm in a new place and find an Op shop, you can guarantee I'm going to have a look. I've even been known to plan my holidays around visits to Op shops further down the country. There's something special about the feel when you find something to buy- could be about embracing recycling, being eco-friendly, finding something to match an already owned set or a walk in the past.
Whatever it is, I see something I love/need/want and if the price is right, I'm happy to support that organisation and make a purchase.
The thing I don't like much is the growing boldness of a few other customers.
Recently, I found a pretty plate in a local charity shop. As I continued to wander around, a complete stranger (another cstomer) confronted me.
She: What have you found? Let me have a look.
Me: It's just a pretty plate I like.
She: It might be valuable- let me look at the mark on the back of it.
At this stage, I turned and walked away. It wasn't anything valuable, just a pretty plate made by an English manufacturer...but what business was it of hers? If it had been a Ming dynasty charger (I'm sure that I would not recognise one if I fell over it, but it sounds flash) would she have wrestled it off me and claimed it as her own purchase/ rushed to the counter and told them to call a museum/ or loudly demand that they put the price up???!!!
Just last week I wandered into an Op shop in Thames. I found a carving I really liked, slightly damaged according to the sticker on it and at a price I was happy with, given the minor imperfections. I didn't really take a lot of notice of the older woman who entered the shop a few minutes after me.
After I had wandered all around the shop and finally to the counter to pay, I found myself standing directly behind the said woman as she was being served. I heard her say out loud "Now, where is she?" She scanned around the shop before finally turning away from the counter and realising there was someone behind her. " Oh, there you are" she said. ( I then realised she must have been stalking me all around the shop and had somehow 'lost' me).
" What have you got there? Let me see" she demanded. I told her it was just a carving that I liked and was going to purchase, without letting her examine it. I then paid for it and left the shop.
What is it about Op shops that lets people think it's ok to accost others and demand to see what they have chosen? Would they do this is in a gift shop/ Harvey Norman's/ any other shop? It's bad manners in my mind and if they get a terse reply (or no reply) to their demand to see what I have found, maybe they should learn from it.
Have others experienced this and how do you respond to being accosted?