Where's your property?
A firm that has sold property for me on the Black Sea (in Nesebar) in the past (disclosure: following that they're also occasional clients of mine too) are Appreciating Assets (AppreciatingAssets.ie) - based in Dublin and Sofia. They're pretty well known in this sector as their business focuses around selling property owned by Irish people to Russian buyers.
If possible, the most important factors for Russian buyers are a seafront location, a building that is modern but not new i.e. a couple of years old so that all the teething problems have been sorted, a pool in complex if possible, and someone who doesn't have unrealistic expectations about prices. A lot of people are cutting their losses, if they got in 2008 onwards then it's difficult to make a profit, but I understand people that got in earlier are still doing OK.
The problem there is that the agents get such big commissions for selling new properties from developers e.g. 12-15%, so that they don't have to touch second properties for which they would only get a couple of percent for, hence you need an agent that specialises in the second hand sector.
Good luck.