Welcome. It's worth considerably more than what Edmunds or Kelly Blue book show, which both show it as fully depreciated. 240sx's are not technically considered "collectible" by dealer source sites as Nissan built a lot of them, BUT, they have become increasingly scarce due to attrition. In addition to age, accidents, and rust, they have become highly coveted especially among the drifting/tuning communities which tend to be rougher on them . And a clean, unmolested, non-rusty, garage kept, single owner 240SX with receipts, especially with a manual transmission would be considered a home run acquisition and could fetch well in excess of $20,000. You might consider getting it detailed nicely and try a national auction site like Bring a trailer.com and set it with a strong reserve price and see what happens.
fwiw, I had a red '89 SE that I bought new as a DD, garage kept for 14 years, and put 180K on it and sold it in 2003 before I became aware of the 240SX's "drift tax" (premium that drifters are willing pay for them). Fast forward almost 20 years, and I'm confident you'll be very happy with the offers you'll get.