spending money is not perceived as correlating with sincerity in this context. Just about any relevant non-monetary act would be better for your purposes.

Yes. People treat money differently than other things, because with money they understand the fungibility: you make trade-offs all the time, whether to buy this thing or that thing -- so it is easy to imagine that any financial decision you did was a similar conscious trade-off.

Other things seem different. On LW we try to understand that they are not so completely different, but for the communication with the non-LW world it is important to remember how they see it.

For example, for a rationalist spending one week creating an anti-war website should be equivalent to spending one week working at some job and paying the money to someone else to create the website. The only thing that matters is the effect of the resulting website. However for most people, spending a week of your time creating the website (unless you are a professional website-maker) signals that you care, while paying someone else does not work this way. (Paying someone else may be a rational decision, but people assume that if you cared, you would prefer to do everything first-hand, even if that would be an irrational decision. This is how people model emotions of others; and this model is rather correct for a non-rationalist.)