Put it on at 0.99. You will often get the listing for free if you do. You can usually estimate the rough selling price by looking at similar items. In my experience an item starting at 0.99 will be more likely to sell and probably for a bit more than the same item at 7.99, even if the item is worth say £10. People want a bargain so they’re tempted by the 99p. People like the buzz of an auction (likely one of the reasons they’re on eBay) so they will bid up and up from 0.99 and it may even go past the 7.99 because they are now invested. Human nature means people want something other people want, bids encourage bids!
Yup it's basically only worth what someone else will pay for it
You may have paid £120 for a dress and never worn it but it's currently worth only what someone else will pay be that £5 £25 £55. You have to have the mindset that it's currently worth zero sitting in your wardrobe Good luck
It may only be worth what someone else will pay for it but there’s no guarantee that it will sell for more than 99p if you put it on for that at auction, especially if your item might be of interest to a very limited number of people who might pay £20, but then only one of them actually remembers to bid. So if it’s worth more to you than that sat in the wardrobe then consider a buy it now. Look at completed / sold prices for similar items then decide.
You can put it on at 99p but also put a reserve price on it. The reserve price can't be seen by the bidders, they just bid as normal but if they don't reach your reserve, it doesn't sell. And I think it's free to do that.
My other tip is that you want your auction to finish at about 7pm on a Sunday because that's the time when the most people are messing around on eBay and you might get a little bidding war going on. And since you want it to be on there for as long as possible, the way to do this is to choose it to start on a Thursday at 7 and then run for 10 days (the maximum time for an auction). You can choose a scheduled start which costs like 10p or if you are selling a load of things, get them all ready to go and then wait till 7 on Thursday to hit the list button.
It depends what it is. Clothing for example, look at what other items in that brand/size are going for. Personally I don't start it at 99p because if there's only 1 or 2 people interested you're going to get very little for it so I put it on for say £5 which I would be happy with.
Some things - Jo Malone, Emma Bridgewater etc go for almost as much as brand new price and if it's a discontinued line then it'll go for more.