Sailing Crew Paying in Advance

[Alexander m]

Hello!
I have contacted a boat owner on find a crew.net
He replied to me and we have agreed on a time and place I will join part of his journey on a recreational shared expenses base.

However he has now mentioned that if I don't want to pay in advance he can't promise my spot if there will be someone else who will want to join on my dates or more and willing to pay in advance.
I understand the reasoning and logic behind this but this my first time doing this kind of thing and wanted to know If that is what happens most of the time and if so , is there a way to protect myself against foul play.

Have you ever heard of schemes the like of this ?
The amount is 350 euro and the sailing will be next month.
I have searched for an answer at the web site for an answer for this but strangely haven't found

Thank you in advance