Jester David
Hero
It's important to point out that this only works if you let it happen. If PCs are trying to sell assets for "tens of thousands of gp" they can't just redeem them for gp at the local shop. It's going to take time to find a willing buyer with that much liquid cash, if one exists. And even then authorities might get suspicious of this group who's constantly selling used ships, or even of non-locals selling a ship for the first time.
Which would be a house rule, as the rules are pretty clear on how much money is available in towns:
http://www.d20pfsrd.com/gamemastering/other-rules/settlements
Plus, when you're dealing with the assumptions merchants have 50,000gp to sell you the equivalent of a +5 sword, what's a few 10,000 sailing ships?
If you do get realistic and question how the party is finding a merchant with magic items worth that much and a collective stock worth hundreds of thousands of gp, then you're denying the players they're expected wealth-by-level of gear and they fall below the power curve. You might as well deny them a feat.