Ah, the Teleport spell description has a failure table with things like "Seen once" on it. if the caster can "Study carefully" they have a pretty good chance, by the rules as written, of landing where they are aiming.
Greater Teleport specifically says you don't even have to have seen it, that a good description will do. Your stated destination has to reflect a clear description that points to a relatively unique location.
You couldn't target "The nearest unguarded Dragon hoard.", for example, but you could say, "100 yards east of the east gate of Rajinpoor, on the south edge of the road.". Presuming that Rajonpoor has an east gate, and a road that leads up to it, those are valid Greater Teleport coordinates. Even if the PCs weren't sure there was such a city.
For both spells, "That room that I'm looking at" would have to qualify. You've seen it.
Now I'm just quoting the letter of the rules, as written. DMs are always free to add house rules as they see fit. That seems to be where you're going, and as long as everyone knows that you're applying a house rule, it's cool.
What isn't cool is when a player or team make plans, based on the rules as written, and then discover, mid execution, that those aren't the rules/
I had a DM I used to play with who had no problem running free and easy with RAW, when it was his guys doing things, then wracking the rules into knots when PCs tried the same thing. He literally couldn't see the problem, or even admit that there were two different sets of rules in play.
Like I said, I used to play in gis game.