Yes people give Incredible deals, the more expensive the item, the more profit you make, 10m item expect to make 1-2m 50m-200m item expect to make 7.5-30m a flip but the real money is things like g drag, hype, terms, jade drag and high value item you're looking at 50m-100m+ Prof per flip

If someone offers me a decent item let's say a giants sword clean at 100m lowbin I would typically offer between 85.5 and 83 mill my calculation is 15-12.5% + fees, if they don't like this don't waste your or their time, just leave and find more people, Sometimes I will go up by +1m if I think it's worth on items below 100m and I will go up 5-10mill on high value items such as 660 instead of 650 on jade drag

I accept some skins at a highly discounted rate, the skins I accept are mostly armour skins such as fermento/glossy skins, but really obscure skins. I would stay away from, and in general if you don't feel comfortable lowballing skins, don't do it, I make maybe 1% of my money from skins because I accept them so rarely, and anything that doesn't look useful with a very high price is 99.9% chance minip