There's probably a ~30% auctioneer's fee on top of the hammer price once you add everything up. Curly at £500 is still pretty good.
£6k sign, huh.
I'm 99% certain that anything to do with severely overpriced art, antiques, and collectables is money laundering. Best way to legalise some drug money, as a lot of these items are close to impossible to value, and take zero effort to produce or pick up at the tip.
Shops selling them have no chance of generating enough revenue to cover high street rent & rates, never mind make a living out of them. Most folks are currently living paycheck to paycheck, so there's no queues of customers forming out of the doors of art/antique shops either.