Draftable isn't getting paid to peddle anecdotes though.
Most of my friends and younger brothers friends have used eBay to sell second hand stuff and buy tickets for various events. I have never heard of Etsy.
I am not a great fan of ebay but if you want to shift something second hand its pretty effective.
A handful of people do not make a trend. However, I feel the comment on eBay being abandoned by under-30's is one of those statements made by someone living in a tech bubble and not in the real world.
eBay isn't hip or cool. That said when you want a good price for something, whether buying or selling doesn't matter. You want a good price and you don't want to get conned. eBay nails both of these.
If she (and you) could demonstrate that as being true (or false) with research and statistics, then she (and you) would have a leg to stand on.