The situation in the US is a little different as you say. The proof laws offer a little safety net and ensure new guns are fit for purpose before they can be sold.
Everything has avalue and it is legitimate to sell any old item. The key issue is disclosure for the responsible seller. The key issue is caveat emptor for the careful buyer.
For example, I will happily buy a thin gun for the right price - I may decide I can do well by sleeving it. If I do not realise it is thin and pay too much for it - then I would be foolish and fools and their money are soon parted.
As a responsible dealer, I would always provide all the data required and advise ignorant customers of the shortcomings of the guns in question, as reflected in their pricing, and try to fit the needs of each customer to the hardware available according to their means or willingness to pay. Not all dealers or private sellers can or will do this.
Therefore, the onus on the buyer is always to educate himself about the equipment he intends to buy (whatever it is) or enrol the help of someone who does to advise him.
I know a little about classic motorcycles but if I were about to spend 10,000 on a Brough Superior, I'd have a friend with me who renovates and races them to pick up all the important stuff I would miss.
