Many units from 2001–2011 have sensors that fail, causing erratic or inaccurate fuel gauge readings.
Before making an offer, check for these common Xterra problems:
Inspect leaf springs for cracks and check for "coil bucket contact" (worn grooves near the front shocks), which happens if a previous owner installed a low-quality lift kit.
Nissan Xterra Reliability and Common Problems - CarParts.com
These models are highly prone to SMOD (Strawberry Milkshake of Death), a radiator defect that can destroy the transmission.