Learn English – What does “of working age” mean

To 'sail the seas' literally means travelling around the world on a sailing ship, but it's also used as a metaphor for participating in something and learning your way around it.

"He was only eighteen when he began to sail the seas of government."

"My grandmother is not ready to sail the seas of social media."

So "Sail the seas of value" (which uses the imperative mood) is a suggestion that you participate in 'value', or less poetically, cheap products.

Since the phrase appears along with the word 'fair' in the description of the book's condition, the seller is telling you that this book isn't in great condition, but is so cheap that it's still good value. It's up to you whether you agree.

The seller's name is Yankee Clipper Books. A clipper is a type of sailing ship, which may be why they've chosen to use this nautical metaphor.

When it comes to signage, alerts, advisory notes etc, certain rules of grammar are ignored. This is the case in most languages and cultures.

"Allow" in this context means to budget for an average or expected amount of money.

If it were to be explained grammatically it would probably say:

You should allow for a minimum spend of $35 per two adults.

It is abbreviated because it is in place of a cost. If something had a very specific price, rather than an estimate like this, you would expect to see something like:

Tickets - $35

... which we understand means "tickets cost $35 each".