There are several things that spell a warning to me but they differ according to how I am approaching a purchase.
The first thing that I sometimes note is when a person slates a particular rifle on a forum, saying it doesn't shoot straight or is rough to cock or leaks but then offers it for sale maybe 3 months later as 'in pristine condition'. Then you have the 'such and such doesn't work but only needs a small part from your local hardware shop to fix'. If it's that easy, get off your butt and fix it then offer the rifle for sale. Generally speaking I prefer to buy f2f.
I am also put off by a seller who is clearly a trader of some sort advertising on line or via magazines. Adverts often come over as being a long term owner but when you see the same contact details month after month and for different products then I begin to wonder and would proceed with caution. If the ad clearly stated 'trader' I would be happier.
For auction buys I am far more relaxed provided I can inspect the rifle beforehand. Major faults can be identified most of the time and a price adjusted accordingly. I am, however, often surprised at how high a price an obviously awful rifles fetches. Buyer Beware! I can even forgive actions secured with self tapping bolts as I can usually replace them with the proper type. Split stocks or bent barrels are more of a warning though. Open sights, or the absence thereof, are a bit of a concern as I generally have no spares and know that such sights can be expensive. I thus prefer rifles complete with sights.
Cheers, Phil