Congrats and welcome to Oldgas!
I would get a metal detector out there. Ya never know, there might be some 6 footers under there.
Stations would sometimes just dig a hole and bury their old stock of signs and other stuff.
Your property probably used to have a Flying A station on or near it and you might have found one of those holes??!
Check out some of the buried signs, posts on here, there have been some very cool finds.
I had one of those TYDOL signs not too long ago that sold around the $400 range. Depending on the condition of them, you could get more or less I would guess.
Dave(509)847-5893 LOOKING-4:WELDING SIGNS,Charter/Finance/PulOil/Trulite/TruTest-Oil,FREEDOM cans,True's gas CORALINE/RAINBOW,FLYING A,Wa/Or/Id Chief!