There hasn't been much demand for 5N.co domains. If you look at
NameBio you'll see that there are only 10 sales reported with the highest being $825 for 88888.co which is a name much better than the two you have.
Most 5N.co sales have been one repeating number (88888/66666) or patterns (51515/11211).. On the bright side, yours do have a pattern somewhat like a couple that have sold.. Both 88222 for $280 plus 90888 for $699 are somewhat similar to your 88855. 76888 for $200 is a bit like your 78888 name. But, to be honest I think you would need some luck to sell these.
If you were to get lucky and find an end user to buy them I think you'd be looking at low $XXX...On the reseller market probably reg. fee tops. There just aren't many sales to warrant a lot of demand for the 5N.co names...