I have no idea, what's the definition of an 'ordinary family'? If I wanted children's chairs then I'd buy these from Argos, 30 quid and a table thrown in, handily on Atlantic Road:
Buy Liberty House Plastic Table & Chairs | Kids tables and chairs | Argos (funky colours ahoy here!) Or I could get them cheaper I'm sure if I scoured Gumtree or Urban75's legendary 'recycle your stuff' forum, on which I have gratefully given and received many things to date.
If a market trader had found some very collectible/vintage designs, and wanted to flog them to make a living at what is considerably below the market rate (admittedly second hand), then is that not OK? If somebody was selling a vintage Gretsch drumkit or Lomo camera at a good price in the Village would you be making the same point?
If some people want to pay £45 for £200+ vintage design chairs a la Philippe Starck, why should they not? It's not clear what you are specifically objecting to. Please be clearer if you can.