Stamp collection

Hi everyone.... I have a bit of an odd question to ask.
First of all.. I'm in Adelaide.
About 6 months ago, I went to a "trash and treasure" sale. I picked up a box of mixed goods for $20.
One of the things inside this box was a BIG collection of stamps.
I took them to my friend's mum,who used to be a stamp collector. She seems to think some of the stamps could be worth quite a bit of money.
My question is where do you take stamps to be assessed? Specifically in Adelaide.
Also..how can I be sure I'm not going to be ripped off? There are literally hundreds of stamps all jumbled up in bags.... I'd never know if one went missing while in someone elses care 🤷‍♀️
How do I approach getting such a large volume of stamps looked at?