It's very poor form, at the least, but I guess not against the rules. If someone wants to pay those prices, that's foolish.

Maybe you could link your original swap thread in a post to her swap thread as an FYI? If it's not against the rules for her to mark up the price so much, I don't see why it would be against the rules for you to let people know that you just sold her those some items for half the price a couple of weeks ago.
That's me with FSG as my styler!!

3b. fine & thick, coarse, dry, medium porosity. Sucks up product and loves ACVs! FSG is my HG!

Currently using:
Low Poo: TJ's Refresh No Poo: CoWash w/VO-5; Sonoma Soap Company sulfate-free poo
RO: Yes to Cuc; SSC Lavender Condish; Sally GVP LI: Donna Marie Miracle D&C
Style: FSG w/Aussie Instant Freeze & CIAB