There are many factors that play a role in deciding the 'viability' of the 'business'. One being total number of domains. One cannot expect 1-2 sale of out of portfolio of 10-15 names, as the above average sales rate is 2-3%. As always, give it some time, would say 2-4 years to realize whether this business is right for you or not. Other factor being 'viability' of the domain itself. Selling them at wholesale prices is not going to cut it. So, you have to think from end user's perspective - that is where the margins are. We normally recommend these 2/3 criteria for deciding whether there is a buyer for the name.
1) Can the domain name be used for a ‘business’. Where does it fit in - product name, service, or business name itself.
2) Does the business identified in Step 1 has large enough scope. Can that business survive of its own by selling that particular product/service. If the scope is wider, meaning there are multiple industries where the domain can potentially be used, better it is as it will have larger audience, hence better chance of finding a buyer.
3) If you answered yes to both above - here is the next step. Since you are doing the domain business, so you are kind of 'business person' yourself. So you have to ask yourself this question, would 'You' run that business yourself (ignore the money part here). And this is what you have to think of when buying the name.
"Be a buyer first before becoming a seller"
Also, what is that important is that you do not have one particular type of names. Have some names that are high yielding, some average, and some that keep the engine running. Keep a mixed bag. Run by a name again and again and do not be fixated in one direction - ask your friends what they think of the name. Normally, people do not realize the value of the name and undersell because they need to sell, but most ask ridiculous prices, more a case of 'hit or miss' (that approach is not recommended). Buyer is no fool, they are businessmen themselves - that is why they are buying the name, and not because they just like the name.
Lastly, in forums, you will see lot of buzz around some names being sold for several thousands, the names that you would not pay $100 for. Do not get disheartened or excited over those news. Those instances are not very common and normally belong to very large portfolio owners, and their strategy is different and is based on their (decades of) experience. You have your own strategy based your understanding and experience.
Hope it helps.