I would never pay someone to decorate your ingame home. Same with buying/selling crowns. There's no function ingame for that kind of transaction, and you have a very high risk of being scammed in doing so.

I specifically laughed and pointed when I saw someone try to advertise their "decorating services" for upwards of 600k, starting out, and topping out at nearly 2m gold.

The only way to prevent yourself from being scammed is to not fall for it in the first place.

There's always a "risk" of getting scammed. Services were never intended to be sold and zenimax has warned that there are no guarantees about services rendered. The only way to lower said risk is to go through reputable vendors, generally people who belong to prestigious guilds whose leadership promotes fair trade. Even then you roll the dice.

FYI other people can't place furnishings into your house, so you can't simply hire someone else to decorate it for you. You could hire them to give you advice on what to get, to craft furnishings for you or to rearrange the ones you place, but you'll need to put the stuff in yourself.

If you're paying them to advise you on what to get and/or to arrange it I recommend agreeing a price up-front (probably based on how much time it's expected to take) and then paying 1/2 ahead of time and 1/2 after. That doesn't guarantee you won't get scammed but it splits the risk for both of you - if they take the money and run or if you were to refuse to pay after the job is done the victim only loses 1/2 what they agreed to. (And remember this is a risk for them too - they don't know you any better than you know them, so they'll be worried about being scammed too.)

If you're asking them to craft furnishings then the conventional fee is for you to provide the materials (or enough gold to buy them at the current market rate) and a small addition for their services. Also, unless you're also paying them to advise you on what to get I strongly recommend being very specific about what you want. "Make me some cool furnishings" is a crafters worst nightmare (next to "make me a good set for this character") because it's hugely subjective. What I think looks cool might be what you think is ugly and boring, or it might simply be absolutely wrong for the house.

And if you are taking their advice spend some time in advance thinking about what you want - not specific pieces but the overall theme - do you want it to look neat? Messy? Expensive? (Literally expensive - items which cost a lot in guild stores, or in-world expensive so lots of gold framed paintings and ornate furniture?) Creepy and cursed? Bright? Dark? Be prepared to spend some time showing them the house you've chosen and discussing what you want from it so they know what to aim for.