Selling 90s old school 10k+ collection but have questions about TCG vs eBay

These are the duals. All the cards in my collection are from 93 to 99 (stronghold was my last buy). They have been sitting in boxes since 1999.

I logged it all into TCGplayer. Current value listed at about $28,800 with my estimated grades.

I have a dealer offering to take it all for $20k pending inspection.

I have noticed that there is a wide discrepancy for cards that are listed on TCG player versus recent eBay sales. Like in some cases hundreds of dollars difference. It’s easily possible that at current eBay sales prices, this collection is well over $30,000. More if I take the time to grade the sweeties.

At $20k for this type of collection, am I leaving too much value on the table?