IS this not already the case? I had a look at a few cases of these last year to find historical sales from 1-3 years ago on b2b market. For example, large pre63 in Clontarf, by the time of sale, vacant possession. Extremely poor condition to say the least. Barely habitable if at all. Asking price was 999k, EA claimed that "restoration to family home" would rise the price to 1.5m. (Owner occupied homes on same street seemed to be getting near that). In the end it sold for 800k. I presume they discounted on the basis of it being on the verge of uninhabitable.That reduction sounds correct. The market will be limited to landlords who are cash buyers as buy to let lending is tiny, literally a handfull of approvals every month. So aside from the price reduction, you might be waiting quite a while to sell. You might also only get one interested landlord who will soon spot that he can name his price.
One of the problems that isn't really getting any interest now is that properties that have been let for now over a decade to same tenants are likely to be in severely degraded states, given that tenants are not obliged for many upkeep items (eg getting the gutters cleaned) so landlords are faced with scenarios in which the condition of the property isn't meeting standards and needs significant investment, making it far more attractive to just sell it.
Don't discount also fact that a huge number of small landlords bought between 2006-2007 and would now only be seeing their properties return to the price they paid for them originally (and in some cases even not).