with regards to cosmetic, if you put time into it, It probably won't sell for more but it will likely sell faster.
IMO the three things that are important for a higher closing rate on sales.
1) overall very clean, no mold, no stains, no oil/sludge in the hull
2) a good coat of "shine" on the interior, fluid film/crc/silicone spray no paint chips missing
3) starts right up on first try when the buyer gets there and idles on its own, no smoke.
shockingly, most of the sellers I tend to deal with violate all 3 rules on a regular basis.
just had a buddy drive 3 hours to pick up a $1300 GSXL and it wouldn't start, dead battery, they dinked around for a half hour while the guy tried to charge it no luck, then he left out of frustration, didn't even bother with a lowball offer. (which was a mistake IMO, I would have offered 3-400, you'd be surprised how often people take those because they're tired of dealing with it)