1)it really depend on stuff and demand..safety level to overup (like +1 to +4 for sqi weapon) is generally same in term of price..for risky overup ,the price is different..for example ,the high overup +10glad(3) really expensive atm because of high demand..the price can always change base on supply-demand
2)for this ,you need to survey market by yourself..every stuff and item can be a tool to play market..you just need to know the demand and actual price..and then ,once you saw and get it for lower price ,sell it at the price that you confident it able to sold..the high demand stuff is the most profitable and efective item to take advantage..if you able to get high demand stuff for lower price ,you can sell it easily at high price