That’s a carp comparison with tesco and baked beans!! Tesco own their shop, pay the cost of running and maintaining the buying environment, so yes it would be entirely unreasonable to put a poster up on their front door.
Sellers on STW use a free service in the public domain. If individuals want to charge their own prices without feedback then maybe they should set up and run their own classifieds site (in the same way as Tesco run their own shops…)!!
And as for profiteering, I’d claim someone is profiteering if they buy an item, have x months of use from it, and then try and sell it for similar to what they paid. Depreciation and wear and tear have a “cost” so it’s not as simple as “I bought it for £50 so now I can sell it for £49”.
I think STW towers should stop being so bloody petty and concentrate more on the numerous real rip-offs and scams in the classifieds.