How do you even put a value on a sound setup the last owner installed?

I'd pull it and sell it piece by piece, personally. I bought a Civic a few years back that came with an amp and sub already mounted, and the seller swore it added value to the deal. In reality nobody shopping for a 20 year old Celica is paying extra for used car audio, and the install itself, no matter how tidy, is basically worth nothing once the car changes hands.

The gear might still have some resale on its own though. Look on the amp and the powered sub for model numbers, then search sold listings on eBay rather than asking prices. That tells you what people actually paid, which is usually a lot less than what sellers hope for. Power storage units and those blue wire kits tend to go cheap because half of that stuff was trendy filler back in the day.

One thing I'd check before driving it much: if that power run to the battery looks homemade, make sure there's a fuse right there at the battery end. An unfused cable rubbing through on sheet metal is how engine bay fires start. That's a two dollar fix if it's missing.

When you price the car for sale or negotiate on it, I'd treat the audio as a bonus you mention but don't count on, and keep the gear in a closet if you ever sell the car stock.

I'm in the keep-it-installed camp, honestly. When I bought my old Corolla years back it came with a similar trunk setup, amp and sub already wired, and I briefly thought about stripping it out to flip the parts. Once I actually looked at what used gear goes for on the secondhand market, after shipping hassle and meeting up with strangers, it barely seemed worth the afternoon of unwiring it all. And then I'd have an empty trunk with holes in the trim and no sound upgrade to show for it. The math only works in the seller's favor if the gear is genuinely high end, like a name brand amp and a sub people actively hunt for. If it's mid-tier or no-name stuff, the value is mostly in the fact that someone else already did the tedious work. Running power cable through the firewall and under trim panels is the part that kills most DIY installs, so a clean existing run has real convenience value to the right buyer even if the components themselves are cheap.

One thing I'd actually do before deciding: check the amp's fuse near the battery and give the whole system a listen at moderate volume. If the sub rattles or the amp cuts out, it's basically a take-it-or-leave-it bonus and I wouldn't factor a cent into the price. If it sounds good, just leave it and enjoy it. That blue cabled power cell thing is a nice conversation piece if nothing else.

Honestly I'd just drive it around for a week and see if you even like how it sounds before deciding anything. I bought a pickup with a sub and amp already in it, planned on flipping the gear, and ended up keeping everything becuase it slapped and I got lazy. Worst case you're out nothing.

Also check what head unit is in the dash. If it's some older single DIN with a bunch of features, that part can actualy hold decent value on its own, more than the amp sometimes. And yeah, if you do pull the gear, don't just yank the wires. Cap the runs properly so you're not leaving bare connectors floating around back there.

Following this because I'm in almost the same boat, just with an old Altima that had a sub and amp in it when I bought it. Half the reason I haven't touched mine is I can't even tell if the gear is any good or just old. I played some bass-heavy songs through it and the trunk panel buzzes like crazy, but I don't know if that means the speaker is damaged or if the car just rattles. Is there an easy way to tell if a sub is blown, or is that a play-it-and-listen kind of thing?