By not buying a kart on Facebook until you know what you need. Resist the urge to buy a kart immediately so that you don’t impulse buy something without being 100% sure it’s what you need.
There are a jillion competing engine packages and a wide variety of chassis, not all of which are appropriate for sprint racing.
Direct sales make sense when you know the gear and your needs. You also need to have the experience to asses the gear. Is the chassis straight? How’s the wear on tubing bottoms? Etc.
If you go to one of the tent programs or other types of sales folks who service the series, you can get used equipment from them that will be appropriate for the series you will be racing in. Since they are local to the series, they will typically stand behind their sales and also be there for service, etc.
Also, bear in mind that there’s resale value in this stuff. So, buy as new and as good as you can. I like to take advantage of the depreciation curve and buy from teams after supernats when they sell off the karts and engines used for the race.
So, first order of business: find out where you will be racing. Talk to people there. Ask them (the race director) where to go get a kart and the like. (In this case, I’d pursue the leads of names given here by the locals).