Both feature parents who disapprove of their child dating the other nationality. Go's female lead's parents don't like Koreans, Friends' male lead's parents don't like him dating a Japanese woman.

Relationship begins the same here, female lead is robbed and follows male lead around aimlessly for help. Plots diverge soon after but I noticed the parallel and couldn't help but compare!

(reccing this as of episode 2 of Doubt airing)
Both feature an arc regarding a child killing a child now grown up, and the mystery of if they are capable of doing so again in adulthood. There is the push and pull of the perpetrator's family wanting to trust believe and understand them, but ultimately not being able to let go of that suspicion.
Both the killers have a stoic and unremorseful personality assisting in the struggle to accept them and their words.

Dong Jae being the spin-off of Stranger, I would suggest watching seasons 1&2 of Stranger first to get a good grasp on his character.

Both center around crime investigative fathers who have strained relationships with their teen daughters. Working on cases that they soon discover their daughter is involved in, they work to hide certain details to protect them while simultaneously searching for the truth.

Gender swapped but very similar story for the most part- rich main character loses memory in an accident and starts to live with other main character while they recuperate (creating messes and causing trouble while doing so). Male leads are both harbouring secrets from the female leads, romcom atmosphere, etc..

Both follow an office-working female lead meeting and later convincing an everyday street-smart type of guy to have a fake relationship with her.

Both star Shin Min Ah, and involve a spontaneous/quick marriage at nearly the beginning of the show.

Door Lock has been a favourite of mine for many years. I'd say it's more of a subtle thriller, where as Don't Buy the Seller (or Target) is more action packed and in your face. I do prefer Door Lock but while watching DBTS I found I was never bored, and quite frankly it seemed like a faster paced version of Door Lock in many ways.

Groundhog day-type. Male leads have to relive the same day (the day their girlfriend dies) over and over after having mistreated her in a way, and desperately try to save her with these extra chances given.

Same circumstance for the male lead- gets into a car accident which leads to him losing his memories and reverting to his childlike self/mindset. Personally I prefer the overall storyline in Spring Day, as the character arcs and romance there were better executed (in my opinion).

Romance takes a backseat in both of these series, but not in an annoying or dull way. The main story and everything surrounding it is riveting, while the loveline helps ground our characters- stopping them from crossing over into insanity.