Opskins, Bitskins, Skinexchange - which would you suggest?

None.

^^
what Ekaros said

I am only using opskins, so I will suggest it

Gameflip is one that I've seen that has one of the best terms for the seller, higher prices, etc. But it takes a while until you sell your items, and you'll have to go for the lowest price possible for a quick sale. Meanwhile OPSkins has an option where you can sell your items instantly, at a reduced price. I suggest checking this two options and see what suits you best. If you don't care much about how much you're getting and just want to get real money without waiting, your best option is OPSkins insta-sell option. If you'd like to get a little bit more for your items, in exchange for some of your patience (it could take weeks or months to sell everything) Gameflip is your best option.

Note: When cashing out your Steam Wallet, you'll usually get around a 65% of your Steam Wallet worth in real world currency. That's just how it is.

I only used OPskins and only as buyer so far. Experienced a few flaws on their page, but didn't encounter issues regarding trading itself.

Sell everything in the steammarket get Wallet
ask your RL-Friends if they want Games for some -% for Paypal
Sell everything in the steammarket get Wallet
buy CSGO/TF2 Keys (buy order) sell them to a trustworthy Buyer for Paypal

but depends on how much your inventory is worth
At some point you always want to buy a game and then you have to pay again with your real money....it is just a cycle
at "small amounts" its only a unnecessary loss sooner or later