It just depends on the set. I've bought and sold probably around 100 sets over the years, been collecting for about 20 years now.
I've had some that I would 'never sell'. Then they appreciate in value, I see something shiny and expensive I want, and sell.
Examples:
Custom Suicide King TRK set. This was one of the last sets made by TR King before they closed. Had no intention of ever selling. Then bought a Jeep. Wanted a new suspension and wasn't hosting much at the time. It had gone up in value enough to mostly cover the parts and labor. So away it goes.
Rick's Moundhouse (brothel) Paulsons (my avatar). Never thought I'd sell these, but didn't use them much in the 10+ years I had them. Someone asked for the price I would let them go. Wanted more parts for my Jeep, and away they went.
When I got my Jeep, and while having it, have probably sold around $25K+ worth of chips to put money into it.
PNY Paulson tournament set: These were $1.30 each for $1 - $500 chips from the original seller. $1000 chips for $2 each. Sold these to buy customs when the prices skyrocketed.
Many other sets: I would buy a new set I liked. My only justification would be to sell another set to cover the cost.
Too many cash sets, sell some of them. Too many solid sets, same. Right now I have enough solids, so 'no more solids' for now.
Bought some sets that looked good in pictures. Then either I didn't like them, or my players complained about them (color matching, etc).
Additionally, I just like using different sets. So will buy a set with intention of using it for a few games, then selling it. This has often led to buying a set, NOT using it, then selling again.
Another reason to sell is if your game outgrows the breakdown. Like a one table tournament set, but you're always doing 2+ tables.
Custom ASM Roman mold: Loved how they looked, didn't like how they handled. Breakdown was outgrown. Got to the point where I had to add on or sell, so sold.