I wonder if the first artist you mention is in fact the Dutch artist Philippe Sadee (1837-1904)? His paintings of boats and ships sell well at auction and sometimes sell for substantial amounts of money. You can get some idea of just how much by researching the artist on the free listings on the findartinfo.com website. K Yoshii is also a listed artist, and you will also be able to get information on the website I have suggested.
As described in the article above, there are many options for selling art. High ticket items often do best at auction, but you can also sell via an art dealer, or via an auction listings site such as eBay. I would personally prefer to go via an auction house if I were the owner of a Sadee, but that must be your decision.