Hi,
I can give you a buyers perspective since I got a new to me big bike via FB Marketplace.
1) I did not take a test ride as I am new to big bikes. Some sellers wanted Cash in hand or money in account if anyone wanted a test ride. One has to be very confident (or stupid) to hand over money for a measly test ride where you can barely scrape the performance of a high end superbike imo. The least I would expect is the potential buyer showing evidence that they are covered on their existing insurance on any motorcycle they ride within UK.
2) Short answer 'Nope'. You can mention you what aftermarket stuff you have added on the Bike and if you still have the originals. You can include them with the bike and the buyer can decide whether they want to remove aftermarket and install originals. You don't really have to do it, its your choice. Please mention what service history you have and any user manuals (Haynes/original) with the bike. If possible, how many owners and how long you have had the bike for.
3) I only did eBay for one bike but I did not bid, only asked for service history which there wasn't any so I bailed. Mostly it was Autotrader but I ventured FB Marketplace as I could see better deals on them for my budget (<£2000). Just like there are stupid low ballers I encountered a stupid seller who called me a scammer when I politely asked for the Reg to check MOT history. (P.S. That bike is still on sale whilst I have had mine for more than a month). Ignore the low ballers and take a safe approach/guess on the potential buyers genuine interest.
4) You won't get a good price at a dealership as it will be evaluated on Trade-in values. Check on WeBuyAnyBike or similar on what price you can get. You will definitely get more on a private sale.
Check Autotrader on prices. I can see two on sale that are for £3499 and £4499 from dealers. Both 2005. Private would be slightly cheaper.
Hope this helps. I am mid 30s so trying to get the average age of a UK Motorcyclist down.