Well it certienly won't go down. 75 seems about average for a sp one. It is a card that will go up over time though since its on the reserve list. If you need the cash it seems fine to sell.
besides the Foil advent wurm(which because its a foil may or may not lose that much value) you dont really have anything that I see tanking in value. most of the expensive cards you have are staples and the others are mostly cheaper that could fluctuate slightly. Always try and trade for deathrite shmanans and abrupt decays though.
B: Foil Sword of Fire and Ice (MM)
thundermaw hellkite is 3RR...
really cant tell without seeing a scan but that seems like a reasonable price
this! I have been testing it as a 1 or 2 of in the board and its also good vs agro to wipe all the two drops off a farseek turn 4