No, I can wrap my mind around being nice and giving someone extra for an item that they are selling at a severely undercut price. What I cant get is the 'HERE IS EVERYTHING IN MY WALLET, TAKE IT!' mentality.
Now, ok they were her son's not hers. It was up to the son to tell her what he wanted and what they were worth. In the long run they weren't her items and we don't even know if she got to keep the money, may have went to the son it was his property after all.
And I see what you mean about comparing it to ebay, but how about this.... Flea markets are a wonderful example. Same as yard sales except there are alot more people there. Your selling the same kinds of goods, you can haggle on price if need be. From the outset it looks exactly the same as a yard sale except there are alot of people at once place inorder to have people see their stuff. Thing is alot of people do it as a career and are there every week but unless your there every week you dont know. I recently got a Airfix 54mm highlander MODEL KIT from a flea market seller that my dad buys army men from. He just doesnt care about model kits and will give them away, sold it to me for a dollar. I didn't argue and go You should charge more! He knew what he was doing, he knew his money was made in other items.
Also when selling OTHER PEOPLE'S STUFF people tend to not really care. I know another lady, we bought about I dont know must be close to 500$ if not more worth of vintage toy soldiers from her. She was selling for someone else and she had piles of stuff on her table. We talked to her, she was just selling it for them, didn't really care to get top dollar for it she just wanted to get rid of it for the buyer. She wasn't a rube, she wasnt dumb and didnt know how to use the internet to price things, she just wanted to sell it and low prices sell. Speaking of she sold everything and hasent been back in a few weeks.
Then there is the fact that the OP is not trying to RESELL these kits and amp. If your buying to sell and pay the really low price if your thinking your taking advantage thats not a good thing, but if your going to use that amp, if your going to build those kits like they were meant to be I see nothing wrong with it.
Sometimes selling the item is all that matters, it isn't how much you can get for it. I'v done alot of yard sales and flea markets in my life. Heck, you know the story about the original copy of the declaration of independence that was found behind the picture frame? Yea we were in that shop the day before the person bought that picture and discovered it. Should he go back and throw his cash at the guy because of the good deal he got on the item?
Oh and lets not mention that times are hard for most people, there are a few threads on here about people loosing their jobs. I know a lot of people out of work and that cant find jobs, myself included. Disposable income is hard to come by now a days and to just go around tossing your money away like that is... ill advised if not down right reckless