i recently bought a daedra heart for 750 gold, i'm not sure of its value but i seemed attracted to it soo what
i want to ask is should i sell or keep and why.

It can be used to make Daedra weapons/armour and it is also used in Alchemy potions.

Just hang onto them. When you do actually need it you'll wish you had it. It can't hurt.

Daedric Hearts are rare and when you want them to make Daedric Armor, you will want every one you can get your hands on.

I have only collected about 6 of these things in more than 200 hours with my character, but I am not looking for them at Alchemy shops, just as dungeon loot.

Hehehe... my Nord.. just lil over 100 hrs has literally 17 Daedra Hearts in the Satchel in Breezehome. I only ever made 1 Daedric weapon and that was a Bow. Also, found out that the Daedra Heart in Jorrvaskr respawns after time.

The 4 Dremora outside and inside the Shrine of Mehrunes Dagon also respawn.

This. ^

I would recommend not using them for Alchemy though because of their rarity.

I came across a daedra heart once at a shop in Solitude. I think maybe once in a dungeon.. not sure. But I've decided that although I'm a really good guy.. I need them sometimes. So I go visit the vigilant's of stendarr and I just drop my pickaxe, then select it on the floor and use it to knock the daedra heart off the table and down the stairs. Then I go in sneak mode, pick it up and walk off. Bad, I know but it works. They always replace it in time. Besides, I help them out now and then. The least they can do is give me one every now and then.

Keep them, never know when you might decide to make Deadric weapon's or armor and If you don't have them, it take's awhile to find them. All my character's have kept them, none so far have made armor, but a couple have made weapons.

Only rare as loot.

I've never had issues finding them, and paying 750 gold for one? Um.... no way.... Every toon I've ever played has dozens of them. In my games they are NOT rare, and no, i am not consoling them. Every alchy shop has them off and on. They can be found various places laying around (check the wiki). Once you do Azura's questline, you can get 4 of them every so often.

The wiki is your friend. Use it.

They are also required in a quest. I'm pretty sure Malacath's quest.