This is a frequent discussion over on the Growing Tomatoes forum here not to mention on all the Exchange forums, and it depends on the trader and what their goals are so it is negotiated in advance. As long as your seeds are labeled properly as pure or OP then no one can complain.

Most who trade actively, if the variety is of particular importance to them, will bag blooms, label their seed as such, and only accept those that were bagged and labeled as true/pure.

Some traders don't care all that much and are willing to accept possible CP seeds as long as it is labeled as such - open pollinated.

Where folks tend to get upset is if they are offered seeds labeled as a specific variety but NOT marked as open pollinated.

If saving for yourself it is your choice. If heavily trading and high quality is a concern for you then bag blooms and ask for the same.

Another issue in trading is if the seeds were disinfected or not and labeled as such.

Dave

Here is a link that might be useful: FAQ - How to prevent cross pollination

Dave:

Thanks for the clarification clearly expressed. You've raised a whole new issue for me; disinfection. Can you point me to some info?

Also, do you know of any cross-border security techniques - x-rays or ?? - that might affect traded seeds?

Can't help with the cross-border issues. AFAIK most just mail them in a standard envelope through the regular mail.

As for disinfection, there are several methods. The FAQ here on How to Save tomato seeds covers one and "disinfect seeds" search on the Tomato forum will pull up discussions about others.

Personally I use the 1:10 chlorine bleach misting method after the seeds have been fermented and just before they are spread out to air dry.

Dave