Steam Card Exchange - Turn in random Cards (if possible), for missing cards (if available). Quick Trade button bypasses loading a 100.000 card inventory!
List of best Sets to get because they produce up to 200 gems / (6 cents €) in recycled background+emote.
CS:GO - 200 gems / 25 credits
https://www.steamcardexchange.net/index.php?inventorygame-appid-730
Dota 2 - 200 gems / 40 credits
https://www.steamcardexchange.net/index.php?inventorygame-appid-570
Prismata - 200 gems / 50 credits
https://www.steamcardexchange.net/index.php?inventorygame-appid-490220
Grimm (120), Kingdom: Classic(160), Minion Masters(200), Company of Heroes 2(160), Deadbreed(200), Starpoint Gemini 2(160), Murderous Pursuits(120), Throne of Lies: Medieval Politics (200), Warframe(200), World of Warships(200), Planetside 2(200), Ball 3D(200), Head Goal(200).

Place for Coupons
https://steamproxy.com/id/TrashDump

SteamTradeMatcher - instant 1:1 trades with a dozen Tradebots or match duplicates with other participants. (Barely usable, now, due to Steam restrictions. Once an extremely helpful resource.)
https://www.steamtradematcher.com/

GTG - Trading bot - 1:1 and cross trades, big collection of 15 card garbage sets. (requires being friends to receive prices and error messages)
http://steamproxy.com/tradeoffer/new/?partner=135203789&token=5opbb4KV

Huge honkin List of 1:1 Bots
https://asf.justarchi.net/STM

Steam Trading Cards
Any set of Cards can be created from booster packs which add up to a total of 2000 gems, which effectively caps the prices for them. (~60 cents currently EUR)
Set of 5 -> BP = 1200<= 36 c
Set of 6 -> BP = 1000<=30 (assumed conversion rate)
Set of 7 -> BP = 857 <= 26 c
Set of 8 -> BP = 750 <= 23 c - Skyrim
Set of 9 -> BP = 667 <= 20 c
Set of 10> BP = 600 <= 18 c
Set of 11> BP = 545 <= 17 c
Set of 12> BP = 500 <= 15 c
Set of 13> BP = 461 <= 14 c
Set of 14> BP = 428 <= 13 c
Set of 15> BP = 400 <= 12 c

The price-range for Booster packs is 3-36 cents. The lowest price for a thing in EUR is 3 cents, thus purchasing the cheapest set via the Marketplace using booster packs would cost at least 5 cents. Bulk sale bots offer (24:1 for TF2 key) 200 / 24 = 8.3 cents a set at the cheapest, though progressively those offerings get more expensive as the sets get more rare (19:1 = 10.5 cents | 14:1 = 14.3 cents).

The gem values of any cards add up and round up to the total gem value of background+emoticon (specifically 200,160,120,80,20).
i.e. (a) A badge of 15 cards worth 1 gem each, adds up to a 10 gem emoticon + 10 gem background.
(b) A badge of 8 cards worth 25 gems each, adds up to 100 gem emoticon + 100 gem background.

100 gems = 3.5 c (net)