Do you mean you put your house on the market for X amount of Euro's and folk viewed it, then offered to buy your house and pay for it, say monthly or weekly directly to you. (Not 100% sure of what you say) ....Never heard of this being done this way in Spain.
You might be desperate to sell along with thousands of others, but do you honestly believe this is the way forwards to sell your house.
If you did do it this way I have no doubt you would get one months payments, perhaps a few more, then what would you do if the buyers stopped paying you, try to explain to a court that you gave them a house on the promise they would pay you back, hells bells some people wont even pay a car loan back of a few thousand Euros or Pounds.
Whats in it for you? The promise that you get to sell your house, paid back over a long term maybe, for the buyer it sounds like an interest free house loan because they have been refused loans everywhere else.
Please don't come back with every check of the buyers back ground and past records will be done, sorry that don't work.
I hope you're not one of these one shot posters with better then daft questions that come on here and go just as quick, and my advice is if you did sell your house this way you deserve everything you might get from the worst way I can think of selling anything.