I speculate there are two reasons why you see so many newer/used 18' boats on the market.
1) Someone bought an 18' boat knowing it would be big enough, then learned they'd love to have more room, and upgraded.
2) Someone bought an 18' boat to start out in the hobby of fishing, ended up not using it much, and is selling it to get out from under it.
That being said, I have an 18' boat, not a bass boat. Very few times I've wished I had more boat, but those few times it would have been very nice to have. Never wished I had a smaller boat, been there, done that. I have fished on many bass boats, and yes even if only two people, the larger the front deck the better it is for all on board. Go with as big a boat as your budget will allow, you won't ever wish you'd gone smaller.
If you're not living on the edge, you're taking up too much space!