The agent is probably referring to the fact that previously, you could "flip" an off-plan property, i.e. sell before completing on it, by effectively cancelling the contract you made with the developer, and your buyer making a new contract with the developer. The difference between the price you bought off-plan, and the price you sell it on at, was profit in your pocket, and the tax man knew nothing about it. However, as always, the tax man caught onto this, and now very few, if any developers will be willing to allow contract flipping in this manner, because they are likely to get into trouble for it. So, if you wish to sell before completion now, it will be dealt with as any other sale, and can potentially (although possibly illegally) end up resulting in transfer tax being charged as well as IVA. This was discussed recently on another thread somewhere. However, the agent is wrong to suggest that it is not an option. But, as Justin said, actually trying to sell it now and even recover what you put in, may be an altogether different matter.
The so-called downturn in prices is an on-going theme of another thread, and makes very interesting reading. Certainly, the £ to € ex-rate is a factor, but purely looking at local prices (i.e. the euro prices), I think it depends very much on the area and type of property you look at. There is no question now that prices are starting to fall in many areas, but others are not as markedly affected - yet. Personally, I think the most notable cases are where prices previously became grossly over-inflated, and a natural correction is now taking place.
5 years ago I was looking to buy in either Torremolinos or Fuengirola. I gave up in Fuengirola, because I couldn't find any value, despite the fact that many local people who have known the areas far longer told me repeatedly that Torremolinos was more expensive than Fuengirola. At the time, they were wrong. I bought in Torremolinos. Today, I see prices in Fuengirola similar to those I found 5 years ago in Torremolinos (i.e. lower than 5 years ago in Fuengirola). Torremolinos prices rose steadily during the last 5 years, and have now stopped rising, and in a few cases, appear to be dropping slightly. But Fuengirola prices seem to have dropped dramatically. Why this should be so, I am not certain, although I think it may be due to a greater number of foreigners buying in the Fuengirola area during the boom, which may have fuelled price hikes there more than in Torremolinos.