Selling stuff should be the last resort. First of all, you lose 50% of the value of the item when you're selling it. That's essentially wasting 50% of the time you invested into this game. You also won't get any money from your upgrades. If you had DHS+Endo on a 'Mech you just sold, say goodbye to roughly 2 million you invested. Secondly, you never know if you'll ever need a 'Mech again. In my case, Locusts seemed completely useless to me back in early 2014. After toying around with them, I sold every single one of them. Now, after all the quirkening they're the tiny ankle biters they are supposed to be. I'm actually regretting selling my Locusts and all of that wasted money. Same with the TDR-9S. It was absolutely the worst heavy I had laid my hands on. Now, it's probably the deadliest IS heavy. The game balance changes, the game changes. You never know what you'll need.
Now, I'm not saying keep everything. Do you have 40 extra MLs lying around? Absolutely sell them. Is there a 'Mech which you haven't touched for months and you really need that mechbay for a shiny new 'Mech? Sell it. Just don't try to raise money for new 'Mechs by purely selling your stuff.