If you encounter her again, walk up behind her and step on her heels. Whoops excuse me, i'm sorry. Seriously though, there is really nothing you can do except use stealth when purchasing around people. Don't let them stare over your shoulder. First and foremost, know your scrap prices and carry a digital scale, I do. Getting gold for $5 is great but it doesn't happen that often. There is nothing wrong with paying 50-60% of current melt for gold. She was wrong for jumping in on your deal, but it's a cutthroat world today at the garage and estate sales. When looking at jewelry, proper etiquette is one person at a time. Granted most people nowadays are uncouth and barely civilized at garage and estate sales, but I will force the issue if someone is crowding me. If someone starts reaching around you while your looking at items in a display case or something, don't be shy, tell 'em to back off and wait their turn. Same goes for looking through boxes. Don't allow someone to start reaching in a box that you are digging through.
PS- If she (the jewelry buyer lady) is offering x amount for certain pieces, chances are it's worth quite a bit more. She is obviously a pro buyer and knows a fair price for gold/silver items. Since it sounds like she didn't pull out a scale and weight these pieces, they were probably worth a minimum of double what she paid for them. I would have played stupid and acted like I didn't know there was any real gold in there. Then I would have offered an extra $10 on top of whatever she was offering, especially since you saw it first.
The next time you see her, cut in on her deal and say "hey those pieces are worth a whole lot more than that, she's trying to rip you off".
