In reality he made a bad choice selling it, he could have sold his level 15 staff for fg (would not have been much) and kept the dagger.
He has no weapons if he unlocks samurai, that dagger would have been ideal for Rogue + Sammy.

But, it is his choice, and you are probably right on the FG amount he got in return, I am not certain about the prices on softcore.

I don't plan to play next class, going to stay longer on rogue.

As for the price of dagger, have sold it for 850fg from which selling fee was deducted, so gain was around 800fg

Good price for first time nice find. Items spawning like that nowadays are quite rare. Hopefully you don't get the bug to climb future classes (sammy/monk/ninja) as this becomes a regret. Good thing though, you may be able to find 80-100 ee daggers much cheaper.

I bought staffs, doing good damage, but I receive insane damage Need to start improving armors now.

Passed to samurai but will run few more climbs as rogue before try that class.
Not sure if I am going to play it, but I assume it has better defense and it's tempting for exploring.

As much as I hate it, its best melee class for you to climb on. I recommend you get tier 3, lvl 5/10 daggers around 75+ ee and as close to 100% ee every 10 levels as you can. Get armours/charms with at least 20-30 dex every 10 levels as well (only armours need to be upgraded for better defense). Higher character classes tend to have more innate magic luck (unless that has been changed) which allows better opportunity to find items than replaying a lower class. Sammy has MUCH better melee abilities with better dps turn around. There are currently swords/daggers up to level 25 with at least 50% Enhanced Effect that could get you started. Most are a bit overpriced imo, but its better than your current plan. At least if you get to MQ as sammy you have a chance at passing to get to paladin which is a good tank/hybrid character with better versatility in weapon usage. Up to you how to play though. You are going to have to invest in gear if you seriously want to play this game anyways, and I'm sure most will recommend to you what I just did, if they haven't already previously in this thread.