Pro advice:
"Do exactly as I say." 😉

Seriously, one of the things that I think first time sellers need to have made very clear is what costs will be involved, when, and how much. Going in. They're going to have to spend money to make money, just like with any investment.
Also, we love our homes, and we're comfortable in them. A buyer will not look at our home the same way we do. We have to look at it through their eyes.
Just do exactly as I say. It will all be fine. LOL

As with any seller, pricing the home right, selecting the right agent, staging the home.

Not stepping back and taking a "good look" at their home's condition and all the imperfections they have been overlooking and gotten used to

Taking emotions out of decisions.

In general, Greeness... like a 1st Time Buyer, they'll need an Agent who can show them the ropes, explain the "why" & give them a play by play as it unfolds. 

Leave the emotion out of the selling....let the Pros handle it and stop listening to outside forces including Google and Zillow etc....

Ignorance and emotions.

I think biggest issue is pricing and net to seller after expenses-especially if the bought just a few years ago. It makes their eyes glass over.

Biggest challenge is accepting change- the market has changed, buyers expectations have changed, the industry has changed,  cost of next home or rent has changed,  and grappling with the emotions of the change

I would say realistic pricing and dealing with emotions and the moving of things that have been collected and stored away over the years. 

There are a lot of things to cover but I would start with education on the entire process and how i can help them. The hope is for them to see me as an expert and trust my advice.

The two issues you mention are directly related to the emotion of the first time seller. My assumption is EMOTION is the subject of the question and the mechanics for which they hire a professional is NOT the subject.

As you suggest, sellers do have a vulnerability that becomes raw when the unexpected occurs.  And it's the unexpected that sends folks off the path and into the ditch.

For me, having a good understanding the real 'WHY' they are selling is important, The 5, 6 and 7, the greater purpose, is the shock absorber when the unexpected occur. Keeping their heart and mind focused on their greater purpose while I take care of the speed bumps keeps everyone on the right path.

Overvalue their property.

The biggest challenge to a seller is choosing the right agent to help them sell it. Some sellers will want to save money and hire a discount agent that could cost them more than the saved. Other sellers may hire "just any" agent and they do not get any service an the advice can be poor at best. My advice is choose a good listing agent, do not over price your home, get your home ready to show, learn what the selling process will entail. If an offer is not acceptable, counter, never just reject them

Biggest challenge is getting the listing priced realistically. If they holler I'm not going to give it away. Explain the valuation and maybe you can not help them if the expectations are not healthy. You set their expectations and give them advice. Fear of "where am I going to live next, I kinda like it here" is part of the feet dragging to not chop chop get into gear in the staging. Seller's remorse can set in. Will they actually sell when it comes right down to signing on the dotted line? Lay out the process, explain the what can go wrong and how we do this, this and this series of steps to get to the real estate closing on time. To make sure nothing goes sideways. Tell them what will keep their house from selling.

Ask them what's more important to them; speed of sale or every dollar of equity. Then price accordingly.

Focus on the wonderful place that is your next address.

Candice A. Donofrio has my answer. 

Most 1st time sellers look like they shoe-horned themselves into the space they have now. Obviously, they want more room & it totally shows. But it shouldn't!

I give them advice on making the home sell faster obviously & ask them not to listen to their parents.

GREED...is the answer to your first question. to your second question, focus getting out of the transaction effectively and efficiently.  

Whether it's one year or fifty years of home ownership it's going to require refamiliarizing yourself with the current market. Understanding what is happening in Real Estate NOW is how to best accept your new challenge. There will be hurdles although I'll make them as low as possible.  

Ditto, many answers before I arrived here to read your question. 

Understanding pricing. 

Facing reality! Their ties are deep and hard to break!

Inna Ivchenko 

Choosing the right Realtor.

It's true! Sellers often pick up the phone and either call a local office for an agent or ask a family friend / relative that rarely sells a home.  Perhaps the worst part is that they hire these folks and actually pay them the same price that they would pay someone who has pulse on local home values, has sold dozens of home in the area over the last year and uses professional marketing techniques to bring the best buyers. 

  Price it right, stage it / present it properly, make it easy to show.  Keep the emotions out of it.  And forget what you saw on HGTV!

Understanding the costs of selling

Others have answered

Getting used to always having their house ready to show.  I always make the point after a couple weeks, it's like that classic Blues Song , "the thrill is gone"

when I was actively listing, I always wanted to first clearly understand their reason for selling.... if it was financial, and I could help them refinance, I did and they did not sell...

other than that, their most difficult task is finding another home and coordinating the sell with the buy...

This is applicable to many sellers, they think that their home is the nicest.  

Being realistic about the value.  Not understanding the process.  Takes a fair amount of hand holding.

Get real...your house is never worth what you think...no one cares about your 10 year old upgrades.

(this is precisely why I choose not to list property)

Eve

Most are remarkably cooprative. They looked at me as the source for directions. It is those who have done it before those who knows it are difficult .....

Inna Ivchenko I think the biggest challenge is the reality that everything has to be sorted out and packed away for the moving company. This can be over whelming especially if the family has lived in the home for several years.

Decluttering.

Nina Hollander nailed this one!

Begin to think of your home as an item for sale.

Understanding that the value is in the eyes of the buyer.

My first timers, it has been their down payment, and having enough to move in and do the repairs they would like.