Buy canon refurb if you are price sensitive, most used lenses tend to hold their value. I declined a $900 offer on my 6 year old one because I decided to keep it. Canon refurb with 15% off is one of the best prices on this lens.
If the seller is motivated and you can get it under $850, that would be a good deal. Reliability is depending on usage, that is the risk you take when buying used.
The 300/4 IS is an old and relatively simple lens design, usually fine for a decade or more. The main concern is whether the lens was damaged or a defect appeared during prior ownership.
