1) You have to be in a "safe" place, like (most but not all) town areas, or on the world map with all enemies defeated
2) Along the water SW of Ipshil, yeah for this kind of stuff you have to explore. Make sure you switch to hover or fly, it's usually WAY faster than walking... (especially after I fill the mech with arc jets, muhahaha)
3) When you find a guy who will buy mechs/mech inventory parts - usually a mechanic, but not all mechanics do, and some non-mechanics might. When you Talk to them, there'll be a "View Mecha" option, sometimes under something else like "I need repairs". Under there you can select a mecha to sell, or to sell inventory from.
btw mechs you're not driving stay in the town you got them or abandoned them, or on the World Map, (or at the bottom of the mines if you already had a mech when you went down). To move them to the current town, look for the ticket guy.
Big tip - if you have a phone you can telephone by jobname if you forget the guy's actual name: T ticket, or T shop for a random shopkeeper in the town, etc. T mechanic, T cook when you're hungry - very handy
