Hello, informed and helpful gamers. I have a number of questions about this game, which seem to be unanswered after extensive searching. I will have more as i progress in the game, and I may add them as time goes on. Thank you so much, in advance!

- combo mechanics: What exactly is the point of these? To allow you to execute an unslotted special within the sequence? Can't you achieve the exact same thing with fast clicking? Is it more powerful/faster (other than saving you a bunch of clicks mid-combat)?

- can you keep an unslotted rune? Guides suggest slotting rather than "eating" a rune, why do this (other than the slotting bonus)?

- What does CTRL+Right Click do, exactly? (Says "Attack or execute special"in the manual)

- Do merchants keep buyback items, or do they disappear when you leave?

- When to use Mentor Potions...higher level?

- Key to various map markers? There are all kinds of icons on the main map, but no map key that I can find. Some are intuitive, some not so much.

-Is there any kind of in-game bestiary, showing resists, or am I limited to the last mob killed? Sure there is a wiki, but it only shows general classes, not specific mob names. I haven't found one, but I find this game has tons of hidden features and mechanics.

I'm not too experienced with the game and it has been a while since I've last played it but I'll try to share what I know.
(i cannot post links so just add http and www before each)

-combos:
I haven't used them extensively but I believe combos are on a different cooldown to your regular combat arts.
I'm pretty sure you can drink yellow potions to speed up combo cooldowns.
It also helps on those characters where you have many many abilities but not enough combat art slots. For example if you played the battle mage and wanted to apply 2 buffs all the time, you'd be freeing up a slot for another type of attack (which might be important later as enemy resistances get more severe and diverse)
Here's a guide I found on combos:
sacredwiki.org/index.php/Combo_(And_Rune)_Guide_by_Bondbug
There's probably more to it, but that's all I could gather.

-rune slotting / eating:
The game determines rune drop rate based on how many you've eaten, if i recall correctly. Because of that, many guides will suggest you go as far as you can with as few combat arts levels as you can before mostly adding just 1 level to each type you plan to use. It's a really poorly thought out mechanic by the developers as it encourages the player to be underpowered. Slotting runes into gear also grants their passive benefits (alongside increasing your combat art level), which is a nice bonus, which isn't applied if you eat them.

- no idea about ctrl + right click, sorry.

-merchants keeping buyback items:
Merchants DO keep items you sell to them (provided they still have space for it), but afaik if you were to re-purchase, it will cost you more than you were given for it. AND it is important to mention that once you visit another merchant, the first merchant's inventory is reset, and your sold items deleted. There's some place that has 2 merchants right next to eachother which some players use to find gear, but I forgot where. I don't know if anything else will reset their inventories.

-when to use mentor potions:
you find mentor potions somewhat often, and if I'm not mistaken, shops sell it too. Because they increase experience earned, there's little reason not to use them all the time, but based on how much money you can spend on it and how many you are finding, you could wait until you're about to do the last hit to a big boss or have just encountered a big group of small enemies to maximize the benefit. You always want to keep a couple on you in case you run into a boss.
I'm by no means an expert on this though.

-map markers:
This might help (top left corner):
sacredwiki.org/index.php5/Sacred:Underworld_Map_of_Ancaria
If there's something else, you could try screenshot or describe it and i might be able to explain.

-I haven't found any bestiary either. When encountering enemies, you can see the resistances to the right of their health bar when the cursor is above them. The dimmer the icon, the weaker that type of resistance is.

Also, not sure if you already know this but, you probably want to reload when you die (so save often). There's a survival bonus on your character page that goes up the longer you are in combat (when the HP ring is visible around your character), which increases your drop chances, and is reset to 0% upon death.
This page also claims it increases your damage, which I didn't know:
sacredwiki.org/index.php/Sacred:Survival_Bonus

A couple of notes. Standard YMMV caveat.

One thing you can use combos for is to store higher level skills: slot some runes into throwaway gear to artificially increase the level of a couple of combat arts, buy the combo, then re-equip your normal gear. The combo art is saved at the geared-up level, leaving you free to gear for other combat arts.

On eating vs. slotting runes: Both eating and slotting will increase the level of the combat art, which increases the recharge time, but slotted runed add less recharge than consumed runes. I forget the exact amount, but I suspect it works out to about half as much.

I don't think that eating runes will make them drop less. I have never seen this even I have noticed that lots of people talk about this in different forums. Reason for some people leaving levels to only one rune is for the very fast recharge time. I personally don't bump up my level with mage for ice shards for example so that I can cast it every few seconds.

One of the best tips I can think of is to explore lot of the map (dark area). This will give you more special items and drops.