@michael 450
I think the key risk in Spain (but also in some other places) is that someone who has access/possession, can turn into "okupas" making no payments. They're difficult to remove, and this is perhaps the main reason why folks are so nervous of your plan.
Plus, I suppose they worry that you might make the problem even worse by granting some/all ownership of the property before you've been fully paid. After all, this is what mortgage companies do.
It's also a lot easier/safer to simply sell to someone who's paying the full price in cash, or has mortgage financing agreed in principle. Your buyers might be lovely people and entirely legit, but... no headache at all is much better than probably no headache, unless it's a really big headache. :-)
However, if you wanted to think of potential contract options, I think the most favourable to you are a pay-monthly purchase contract, or a rent-to-buy contract. They're pretty similar, and in both cases you make sure you are NOT transferring legal title (at the notary) until you are receiving the agreed funds.
For a pay-monthly (a form of "seller financing"), you agree a selling price and a number of years to make the total payment (initial deposit + monthly payments + balloon payment = selling price). You go the notary ONLY when the buyer has the remaining balloon payment available to fulfill the contract. If the buyer doesn't pay the agreed monthly installments, or doesn't have the balloon within the agreed time frame, then he loses the contract/property and you keep all payments made. Note, you have to allow for interest if you give them several years... otherwise you're getting much less money than you think. (i.e. selling for 100k today is much more valuable than receiving 100k in 5 years).
The rent-to-buy contract is simply a rental contract in the usual way (one/two month rent as deposit + monthly rent), but with an agreed purchase option clause. This extra clause says they have the right to purchase the property for X euros (either in cash or with mortgage financing) at any time in the next Y years. Again, you have to consider the impact of lost interest if you give them 3 or 4 years, and you also have to agree how much of the rent is credited towards the purchase (e.g. 0% - rent is just rent whether they buy or not, 100% - rent is just rent if they don't buy, but entirely credited against the selling price if they do buy.)
Both of these contracts avoid you giving ANY ownership of the property until you are paid in full.
Personally, I would lean towards the pay-monthly as I could ask for a bigger deposit. Anyone who gives you 30% down has made a significant commitment, and losing this because they breached their contract (by stopping payments and turning into okupas) would be very painful for them too (not just for me). However, if things went wrong for them (e.g. lost their jobs and couldn't keep up the monthly payments) I'd imagine they'd stay as long as they possibly could! Even so, the cost of evicting them would likely be far less than what they had paid you.
