there are 2 types. While both can give dota 2 beta access, the key cannot be traded through steam inventory. If you look in your inventory and you see Dota 2 there and tag says 'tradable' then you can trade it via steam inventory trading.
If it's just they key, you can only invite someone from your friends list to play Dota 2 and they don't have the option to store it as a gift in their inventory. So was asking which one you had. Can provide a screenshot if you are still confused.
il make you a good offer, add me and we talk http://steamcommunity.com/id/bekiy/
I can offer you anything you want from here - http://www.steamtrades.com/trade/WtPYh/h-games-coupons-coal-everything-you-need
Interested in any of those for dota2?
Braid, Cortex Command, Machinarium, Osmos and Revenge of the Titans
VVVVVV, Crayon Physics Deluxe, Cogs, And Yet It Moves, and Hammerfight
Super Meat Boy, Shank, Jamestown, Bit.Trip Runner, and NightSky
Darwinia, DEFCON, Uplink, and Multiwinia
Magicka
Ben There, Dan That! and Time Gentlemen, Please!
Dungeons of Dredmor
Fate of the World
Gish
Hacker Evolution:Duality
Hacker Evolution: Untold
inMomentum
NightSky
Saira
Scoregasm
Spacechem
Your Doodles Are Bugged!
Zombie Shooter 1
Zombie Shooter 2
^Look at the title.
Dota 2 in my inventory.
Because I can't trade it you'll have to go first. No, I'm not going to scam you. Me on steamgifts.
https://steamcommunity.com/id/monolight