Well, I'm not a lawyer but as I understand it the law is The General Product Safety Regulations 2005. The government helpfully provide the full text here
https://www.legislation.gov.uk/uksi/.../contents/made.
The law places most of the legal obligations on 'producers' so we need to know who those people are. Fortunately Provision 2 defines a producer:
“producer” means—
(a) the manufacturer of a product, when he is established in a Member State and any other person presenting himself as the manufacturer by affixing to the product his name, trade mark or other distinctive mark, or the person who reconditions the product;
(b) when the manufacturer is not established in a Member State—
(i) if he has a representative established in a Member State, the representative,
(ii) in any other case, the importer of the product from a state that is not a Member State into a Member State;
(c) other professionals in the supply chain, insofar as their activities may affect the safety properties of a product;
(my bold). In section (c) they talk about professionals. But in section (a), which I think applies here, they don't. They talk about persons. The OP is a person, so I think that means he's a producer.
The core of the regulations come in Provision 5, which says:
(1) No producer shall place a product on the market unless the product is a safe product.
(2) No producer shall offer or agree to place a product on the market or expose or possess a product for placing on the market unless the product is a safe product.
(3) No producer shall offer or agree to supply a product or expose or possess a product for supply unless the product is a safe product.
(4) No producer shall supply a product unless the product is a safe product.
I understand 'placing a product on the market' to include offering it for sale.
So there we are. I've had a look through the regulations, which are the law, and I can't find an exemption for private individuals - it seems to apply universally to 'persons' - but if you can then it would be useful to see it.
On a more positive note, Provision 4 says:
These Regulations do not apply to a second-hand product supplied as a product to be repaired or reconditioned prior to being used, provided the supplier clearly informs the person to whom he supplies the product to that effect.
So that looks like a possible route for selling.
Cheers,
GJ