Ah, this was my very first magic item. I've had so much fun using this thing.

Quote from Eldritch_Queen >>

Ah, this was my very first magic item. I've had so much fun using this thing.

 My first two :-) 

"Proficiency with a longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it." Isn't it wrong?

Your attack roll is the roll to hit. So this is saying if you are proficient in longswords you are also proficient in Flame Tongue.

But what if the effect is on, let's say Rapier? Do you still need Long sword profficiency then?

Quote from gaber11979 >>

Quote from Eldritch_Queen >>

Ah, this was my very first magic item. I've had so much fun using this thing.

 My first two :-) 

 mine too, in my first game i played a dragonborn paladin and cast searing smite on it to slice a centaur in half and dealt double fire damage

Ah! Excellent question Mr_Rav! The text would change to read "Proficiency with a Rapier allows you to add your proficiency bonus to the attack roll for any attack you make with it" if you decide to make Flame Tongue take the form of a Rapier.

How much gold would this sell for in a shop

Depends on what the DM decides tbh, but if you wanted to sell it to someone, then it could be anywhere between 5-10k gold I believe

When I DM, I also consider how hard it would be to find a customer who is willing to pay that much and has the money available.

even though it doesn't say it is, is this a +1 item because its magic or is that a property separate from being a magic sword?

Hobbton:

The +1 property isn't inherent to magic weapons. It would be a separate property for an item. The DM could decide to alter it down the road.  According to the Rules as Written, magic items are usually a lot more durable than their mundane counterparts.

Quote from Hobbton >>

even though it doesn't say it is, is this a +1 item because its magic or is that a property separate from being a magic sword?

Hobbton:

The +1 property isn't inherent to magic weapons. It would be a separate property for an item. The DM could decide to alter it down the road.  According to the Rules as Written, magic items are usually a lot more durable than their mundane counterparts.

A Rapier is a martial weapon. If you are proficient with martial weapons(or picked the weapon master feat and choose Rapier) then you add your proficiency bonus

holydasdasd as 

 as a dm pricewise for stuff like this i take the price of the item then tack on another amount of gold pieces depending on the rarity

What does it mean by requires attunement

So in terms of role play... how does the player learn the command word? Trial and error? Magically via Attunement? After a long quest? Paying off a forge cleric or an artificer?  Love suggestions 

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Quote from Crossing353 >>

What does it mean by requires attunement

It means when taking a long rest you connect with the weapon becoming apart of it to you. U can only use the true power of the item if you attune to it.(focusing your energy on the weapon)