Hey, just started to get back into my card collection from when I was a kid. After cataloging all my cards and separating them into their respective sets, I am left with a bunch of sets where I am missing anywhere from 2-20 cards for WOTC set releases. I am wondering whether people mostly just sell their incomplete sets as is, or invest in buying the missing cards and then selling as a full set. Naturally the cards I am missing are generally the holo rares and am not sure if it is worth investing into the market for those cards just to sell them right away. Appreciate your responses 
It’s really a choice of preference. If you’ll enjoy spending the time and effort sourcing the cards to complete the sets then I suggest you take it up as a hobby.
As you’ve suggested that you’re mostly missing holos it will likely cost more to complete the sets than the cards you have combined. But if you don’t have the time and money to invest in completing the sets there’s nothing wrong with listing them as part complete or if you have complete commons & complete uncommons you could split them into those categories then sell the rares and holos individually.
But it’s a matter of preference, I hope you enjoy if whatever you decide to do
@sg27 summed it up pretty nicely.
In my opinion, you should consider trying to find the cards you are missing by just searching for deals on said cards from time to time. I completed the sets I collected a while back and cashed out, and though it was a lengthy process, I thoroughly enjoyed it. If you do choose to embark on this collection journey, it doesn’t have to happen overnight as you can take your time to enjoy the process and in the end have many good memories of the journey as well as turning a nice profit. Overall, the choice is yours but I’d say to give it a shot…and if it doesn’t work out and ya cant find any good deals or experience lack of time/money, you can always cash out.
Either way, I wish you the best of luck in your collecting journey!
If you just want to get rid of them it is probably not worth the effort to try and complete the sets before selling. Especially since many old WOTC cards are at record highs in prices and the cards you need to complete the sets are mostly the more expensive holos.
I think your best bet is to sell all the extra cards you don’t want as a collection/lot on ebay or locally. If not, than just sell the holos and don’t worry to much about the commons unless it’s 1st ed base.