Any car/truck that has been customized is going to be worth a good bit less than a clean stock truck. Unless it has all the customizations you want or need, or that you would do, then it may be worth more to you, but don't tell them that. Use KBB and Edmunds to get a good baseline then negotiate down from there.
The issues are, you never know much about the customizations, who did it and what corners were cut and etc. Have it inspected. In the past, I have brought cars home to hand wash them, this gives you a close up inspection of the paint, the undercarriage and etc. I have found paint issues in the past, evidence of accidents and abuse.
Remember, you can't loose what you don't have, so look at like a business deal until you sign the paperwork. Don't get emotional or you will loose.
But, it looks like a nice truck.