I'm thinking of buying antique fountain pen, but the barrel is stuck to the nib and section because ink has dried it closed.
No. Dried ink is not why it's stuck together.
With vintage fountain pens, the section has been glued to the barrel with resin. The best technique is to use a heat gun (or hair dryer) to gently heat the barrel/section overlap area until the resin softens enough for you to separate/unscrew the section from the barrel with your fingers. It doesn't take a lot of heat, so start slow and gradually test for sufficient softness in 10-15 second increments your first time, until you get a feel for it.
Do not do the Frank Dubiel "Da Book" thing of using an open flame. He was a labtech and knew how to control heat with a bunsen burner. I, as a once-inexperienced vintage fountain pen collector, can personally attest that you can melt pen barrels over a candle until they're like taffy. The glass-nibbed Japanese celluloid pens (e.g., Spors) can even combust. Please don't ask me how I know that. [facepalm].
The nib and feed may be stuck in the section due to dried ink. The nib and feed are always friction fit into the section because airflow is required for the capillary exchange of ink/air so the pen can write. And if regular fountain pen ink was used, it's just water-soluble dye, so should dissipate with water. But. If India ink was used by accident, the pen is more or less ruined, because any solvents that can dissolve the ink are likely to dissolve the plastic of the feed/section as well.
Do not soak the entire pen because water can contact metal internal parts (such as the J-bar (press bar in your illustration) if it's a sac-filler) and cause rust. Prolonged soaking of hard rubber causes discoloration (black becomes gray). I only do an overnight soak of the nib, section and feed unit after it's off the barrel, and then knock the nib and feed out using a nib block, if I can't just pull them out with my fingers.
A nib block (aka knock out block) is a solid object with a hole drilled through it, where the hole is small enough to securely keep the section from entering, but large enough for the nib and feed to fall through. Using a punch and lightly tapping on the rear end of the feed to push it forward from the back end of section will separate a stubborn feed and nib from the section. Pendemonium, as well as other online vintage pen dealers, offer tools like this, or more specialized tools (like a vac wrench for Parker Vacumatic sac replacement) if you can't DIY your own.
I also feel I should state that the most common issue with vintage pen restoration is replacing rubber bits that have fossilized or gone sticky/soft. So, you will probably want to have a link to the Pen Sac Company, and a source for o-rings and gaskets for your specific model of pen; vintagepens.com is one good source.
See also David Nishimura's article on Pen Repair Do's and Don'ts.