I think it should be common courtesy to thank the person for sending photos of something you have for sale or trade, not to is just plain rude. It is no harder to say thanks as it is to ask for photos.

I agree.  I posted about this before...  It was eventually shut down.

I agree.  I know I have been guilty of this in the past.  Trying to do better.
Thanks for the encouragement.

I try to always thank the person for sending pictures, sometimes (too often I fear) it is with a check for the purchase of said item.

In fact I'm very cautious about asking for pictures unless I'm very serious about buying. I don't want to take people's time just entertaining me with more pretty pictures.

What bowwild said.

Though I'll admit sometimes its the pics that send me over the edge into purchasing.  A pic's worth 1000 words  :D   that's why posting pics to ads or at least a link to pics is so important.

But thanks are definitely called for, purchase or not.

I'm old and try not to seem too old, but it's a shame that this has to be pointed out

Am I wrong to thank them in advance, before receiving photos then?

This thread has run many times guys. There are and always will be tire kickers.