'Worth' is just about the most subjective thing in the world. But classified ads vary massively, as with anything. Different forums have different sorts of folks, similar things with facebook groups, varies by item type as mentioned etc.
IMHO some people too often expect, heck demand, things be insanely and unjustifiably cheap just because it's 2nd hand which apparently means it's now nigh-on worthless; then throw their teddies out if a seller does anything different.. as if choosing to just not buy the item isn't always an option. That said, yes there are a few (generally quite young) who don't quite understand the market and do try to sell low quality goods for sometimes more than they cost originally despite said goods being used and/or messed about with. But I think if anything the thread linked shows that's the small minority of cases, not the other way around. Where this often gets messy is when people have 'upgraded' a gun, which is a misnomer because most aftermarket parts are no better than stock parts, so don't do anything to the value of your gun. If you've changed a gun around at all (inside or out) it's generally best to just keep it. Alternatively if you've got some high-value externals bolted on to something, replace them with the stock parts then sell the gun and sell those accessories seperately.
I've not really looked at the RIF section on here, but I know on Arnies you'll see guns advertised initially for a good bit less than RRP, then sellers will incrementally drop prices over time. Since there generally isn't that great of a demand for 2nd hand RIFs (certainly not at the mid to high range prices), the eventual selling price will often end up being very low after many of said incremental price drops. My personal take is that if someone's selling something that's literally in the same new condition as they got it from the shop, but they're only asking say £10 less than RRP and it's an item I'm interested in, then that to me is a great opportunity to make a saving and I'll take it. Generally the done thing is to look at the price asked and if you're interested, come in with an offer maybe slightly under what's being asked and most folks will go with that. I know when I'm selling I'm always open to negotiate a little as long as it's reasonable. Obviously what you're offering needs to be proportional however; £15 less on a £250 gun is fine but £15 less on a £30 shirt is being a bit ridiculous.
Coming up with values for stuff (unless it's simply in never been used condition) is a bit of a dark art end of the day, because this isn't bargain hunt and there's no expert to figure it out for you. You have to look at what you paid, current retail price, condition of the item, market demand and various other factors. I know I've misjudged and gone the wrong way in both directions before now. I'll tend to make a fair estimate range, then initially advertise at the upper end of said range as quite often I've gone too low and missed out; you'll know when this happens as you'll get multiple messages from folks asking to buy at your first advertised price right away with no haggling.
My personal peeve is "What's your best price on this?", because I've already posted the price that I'm after, if I had a better price in mind I would've posted that. If you want to come to me with a reasonable offer on something I'll quite probably say yes in order to get the item sold, but maybe do a little back-and-forth depending on the conditions at hand.
The fact with classifieds is you can save money by investing time in searching, but you need to do a lot of searching to find exactly what you're after and that can and will take lots of time generally. You need to be on all the forums, any facebook groups and trawling ebay for gear on a pretty much daily basis sometimes. I know I have certain ebay searches bookmarked as I'm doing them daily trying to hunt down rare items that very rarely come up for sale in my size and at a price I can afford, so being able to get the results in one click is just incredibly handy.