I inform you that your capital gains tax is assessed regardless of your nationality. Only the issue of residency for tax purposes is taken into consideration. A non resident will pay 35% on the net gain after selling, as opposed to a resident, which will pay 20% on the net gain. Also, a non resident will have a 5% of the sale price withheld by the buyer on account of the capital gains tax. This obligation on the buyer was implemented to avoid tax loss from non residents which would sell and not pay tax. After the tax liability is assesed, a refund may be available, as the case may be.
It is common in Spain to under declare the real selling price to mitigate the captital gains tax liabilities: vendors tend to impose that payment will be by draft or cheque and a fraction in cash. This practice, although illegal, is quite normal.
Other than that, if you are resident in Spain, you will pay taxes for your worldwide income. If not, only for the income derived from a business in Spain, plus property taxes, which also differ slightly from the equivalent taxes paid by Spanish residents.
