People don’t like paying for a service
Sorry to tell you, but you will be fighting an uphill battle. As I said in another topic: People are very willing to pay for products, but are not very willing to pay for a service. (See topic Furnishing Services .)
- People don’t understand why it takes you 10, 20, 40+ … hours to source and chose a few nice products. “Why waste my money?”
- They don’t respect your work: “I don’t need anyone tell me how to spend my money.” or in other words “The only thing you will do is spend money and not add anything else.”
- They don’t value your work correctly “Why would I pay you for something I can do myself?”
Bad moment
Other problem for designers, unlike e.a. real estate agents, you are coming in the moment that all money is only going out. In the case of AirBnB hosts: Some start BnB’ing from their own home already in search of extra income. Others just invested in a property to BnB, and maybe already have to worry about how to pay the mortgage.
AirBnB = uncertainty
There is a lot of uncertainty in AirBnB’ing so people are reluctant to invest a lot of money. Quite a few hosts on this forum have told stories of how they source things at very low cost (overstock, second hand, …) or even for free (e.a. mattresses). This is a service you will not be able to provide, because your time would cost too much.
Extra investment does not always equal extra income
If I recall well, people have commented on this forum that extra investments haven’t always resulted in extra income. This due to many reasons, but in any case it’s something YOU can’t control. Therefor it’s also uncertain if you indeed will add value.
Regional and other restrictions of your service
If you are located in Spain, what kind of products can you source for someone in e.a. Belgium, USA, Peru?
How can you provide a plan if you can’t go onsite to measure?
On another note
I’m convinced of the added value of designers 
, but I’m not the one to be convinced. You will have to decide how you will market your service and what kind of service you can and want to offer. For AirBnB hosts I would argue that most will not want to pay you an awful lot of money. As for this forum, hosts can already find a lot of free advice (also on design 
).