Being in the UK is a bit different than here,, But you do have some great swap meets (Autojumbles) and some enjoy such endeavors. For some its socializing, A presbyterian minister was once talking about "Fellowship" and basically hanging out with other believers and sharing your faith. I see club events and swap meets/jumbles much the same. Some, its the same junk every event they drag out, Others its move it out.
I have been in the boat of doing old cars and bikes as a business, but my problem is I am more of an enthusiast than a business person, so I cant say I was ever very successful with the $$ end of it. (Mrs Sluggo is more forceful in her opinion on that). However I do find the super aggressive wheeler dealer types super annoying. I will turn down filthy Lucre from a profiteer to sell to a true enthusiast, as well as I refuse to sell and will interfere with others who will part out a bike (Classics & vintage, I dont care about modern), Butchers and killers of cool bikes.
Many of us here in the US use Craigslist and not as popular outside here,, It has its own special form of weirdos, But I often connect with good people buying and selling. (Just bought a large pile of fence posts for my farm on there, and some parts for my tractor) You have to employ some skills to filter out the BS.
I used to enjoy selling on ebay and did so for others and businesses as well. Not as much anymore and I could rant for hours on their current practices both policy and what results occur because of the changed enviro. However it still kinda-sorta works in a way. Its still a huge global platform. One MAJOR change is, not for parts, but for cars-motorcycles-boats and such, Its no longer binding contract to complete a sale, Which creates real problems IMHO. I found this out when I bought a BSA project bike a few years back. The seller reneged on the deal as he didnt net as much $$ as he was expecting. I found out it was a policy change, so IMHO it severely undermines the sites credibility and I see many people just using it to steer a deal or business to off-eBay. For example,, On your frame,, it would still be a part, so not as much of a problem, but a whole bike is different.
Be advised,, at least here in the US, ebay NOW charges Final value fees not just on the transaction (% of sales price) but they ALSO charge on the shipping costs as a %. This requires significant changes in strategy.
Im a packrat, (hoarder) and I cant bear to scrap something useful, so often times I dont care if I made any money on some items, I just want it to go to someone who can use it. For example I sold a Russian Ural frame and sidecar bits,, I was correct that some weirdo wanted it and listed it for $19.99 and it did sell. However the shipping, even truck freight was quite expensive. Guy paid way more in shipping than the purchase price. I was happy-so was he. Win-Win But under the current policies,, It would have COST me and left me deep in the red to get rid of it. (Fees exceed the income)
In other cases, Some simply do not want to be bothered with the whole ordeal. I have sold stuff for others for a nominal fee, and while I dont seek out such business anymore,, I know those who do, but not in your area but they do exist. Sometimes its worth it to pay a little to have someone else deal with it.
One last bit of advice, Again, I dont know about the UK, But Paypal has amended their policies on sales. On sales of items, whether off a website, ebay, good buddy deal if you indicate its for a sale, the buyer can opt to ask for a refund/return for ANY reason up to 6 months later. (including found it cheaper elsewhere) this is distressing as I have minimal cash flow these days and cant absorb an unexpected demand for $$$, So, You will often see people do Friends & family also to avoid paying fees on the transaction. That is risky, If there IS a legit issue and the seller refuses to make good, you have zero recourse. But with people you have trust with, its an option.
Anyrate, thats my take on the whole buying and selling on the internet, I do like to deal with people on forums, whether its a car-tractor-farming or motorcycle forum, its easier to spot the crazies and most are true enthusiasts and I have many internet friends around the world by connecting with them via our interests.