I think you should give it a go and see how it goes, you'd definitely get a lot of interest from those wanting a cheap job and only trying it out will show what level of interest you get from what kinds of manufacturers.
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If the site takes a cut it will have some level of responsibility (whether it wants it or not) for ensuring the competence of it's registered "manufacturers", you'll need some serious commercial insurance in the event that a part sourced through the site fails and the customer sees it as a better bet to sue you than a hobbyist working out of a shed at a rented address.
A disclaimer will not stop someone taking out proceedings against you which could cost you your house before things get anywhere near a courtroom, a Limited Company is no longer good protection as direct action in person against directors and officers of a company is now commonplace,
JMTP,
- Nick