Sounds like you got rusted thottle cables. Lube them with WD-40 and see if it helps.
For maintanance sake, its better to lube cables with something like chain wax, its not as slick, but it lasts much longer. However, if the cable is sticking, WD-40 will do the trick.
If you can, get a cable luber tool. They are pretty cheap (less than $10). Not required, but make the job easier. Disconect one end of the cable, insert into tool, hook up lube and spray untill lube starts coming out the other side. It will likely be abit messy, so dont wear your good shirt.
If at all possible, try disconnecting the cables all together. This will allow you to check that the throttle bodies close smoothly (there is a possibility that the spring is worn, but I doubt it), the handle doesnt bind (I never seen one that does, but it is possible. If it does, its more likely to have some forign crap in it than be defective. Dissasemble and clean) and that the cable can more freely by themselves)
Also if your cable are too tight, they will try to bind. You should have an adjuster on one of the cables. You should have a little bit of play in the throttle. Not much, maybe 5-10 degrees.
One more. Since its a used bike, check that you havent had a faster throttle installed (turns the throttle from 90deg to 60 or 45) those will give greater leverage to the return spring and make the throttle harder to turn.. and well, unless you are a pro, they are pretty useless to you. If you want to keep it, you really need a new spring. Otherwise your hand will seriously hurt after a long trip.