sell your property without capital gains tax

[costa-brava]

Believe it or not, this appears to be possible but there are two or three key conditions. It has to be your "vivienda habitual" (that is to say the house or flat you have lived in for at least two years. The sellers must be over 65 and must continue to be tax-resident in Spain. Under these conditions you are considered exempt.
What it means is that if you have your own house and a holiday-home, it can be more advantageous to sell your own house and move into your holiday house (or one of your holiday homes).
Of course this assumes that you have made a capital gain on the sale.

[Casscat]

You only have to be tax resident in the year of sale - there is no commitment to remain in Spain thereafter. Under 65s can also avoid CGT provided they reinvest their sale proceeds in another EU/EEA property within two years.

[costa-brava]

An important clarification. Exemption in Spanish taxation is a wee bit wooly. In the circumstance I described you HAVE TO declare the gains you have made and you are SUBJECT to the tax but then by another clause in the law you are exempt. The critical factor lies in keeping the tax office notified of your fiscal residence. You have to notify them at the time of selling and moving to a new address. This of course applies in any case regardless of what you are doing but it is even more important for the exemption. If you just don't declare it you will get a hefty penalty.