If you're planning on playing Fortnite heavily, it's generally more cost-effective to go with the Battle Pass (950 V-Bucks or ~$9.50 US without any Tier Buyouts) as you get 2 skins with purchase (at Tier I) & applies retroactively for the season (so you get all the rewards for all the Tiers you've reached at that point). HOWEVER, with Season 4 ending on July 9th (in roughly 3 weeks, at time of post)... the value may be a bit limited.
By skins in the item store is a bit more expensive (as items vary between 500 & 2,000 V-Bucks... or roughly $5 - $20 US equivalent, depending on rarity of the item), but you can get EXACTLY what you want without the grind (or extra costs of tier buy-outs).
ULTIMATELY, it's more of a question of YOUR TIME (by grinding levels & tiers) OR YOUR MONEY (with tier buy-outs & individual item purchases) here.
As for buying accounts, THIS IS AGAINST EPIC GAME'S EULA (& TOS) & will result in an INSTANT, PERMANENT BAN of the account(s) in question since Epic Games owns all the content of Fortnite, which they grant a license for players to play the game. Therefore, selling the account is the legal equivalence of fraud (as they are selling something they do not legally own NOR are permitted to sell by the rightful owner)... hence why Epic Games bans the accounts being sold. Since the account sellers know they're breaking the law, it's does stand to reason they won't commit other illegal activities as well (the common paradigm is "No Honor Among Thieves") Therefore, it's best NOT to partake in such activities.
While it's you decision what to do with your time & funds, I hope this sheds some light on the subject.