I have a policy of never turning down a job if it is offered to me personally. Now some employers "invite" 100-700 people to apply for their jobs. I do not consider this a personal invitation, so I frequently will decline these. However, I always try to give the employer a note explaining why I am declining.
When a person sends me and only 10 other people an invitation, I frequently bid on these and if I am contacted personally through the messaging system on Guru, I definitely bid on it. I do not always get these jobs, but usually it comes down to a cost thing. If I am busy and they want it quickly, it will usually cost more than if I am available and they do not care how long I will take to do it.
Sometimes people will haggle with me to get my price down. These people are frequently from another country where this is a common factor. However, sometimes I have not understood the project. For example, one person wanted me to write a script about the negative effects of this comprehensive immigration bill that is before the Senate. I initially thought he wanted a 30 minute detailed movie and bid on it as such. He contacted me, though, and informed me he only wanted a 3 - 4 minute script for YouTube. Needless to say, my price dropped drastically, and I was hired.