Hi,
You can save the seeds and still trade, just tell traders that they are open pollinated and there is a possibility of cross-pollination. Most tomatoes have blossoms that fertilize themselves and don't cross easily. There are a few exceptions. And if a very rare variety, you may want to have more distance to insure purity. Of course you can bag the blossoms too. The link below gives more indepth info.
Remy

Here is a link that might be useful: Seed Saving Tomato FAQ

Shutterbugsyl, you are right to be concerned; I grow a lot of tomatoes from seed swaps, and crosses are a frequent problem. You are thinking ahead - which is good on you! There is a difference between those who save seed strictly for their own use... and those who intend to share with others, who should take extra precautions to ensure purity.

Remy's link is one of the better ones on GW, since it covers all the bases - with illustrations. If you are growing large numbers of tomatoes in close proximity & saving seed, blossom bagging is your best option. You only need to bag one or two clusters per plant (or per variety), and Remay or Agribon floating row cover is easy & economical for this purpose.

When the tomatoes begin to enlarge within the bag:
(1) Check for holes in the bag caused by insects; if you find one, do not save seed from that cluster.
(2) Pinch off any remaining flowers or buds.
(3) Mark the cluster with something that is bright and not easily removed (birds love string!). The marker should be easily visible when the tomatoes are full-grown, to avoid accidentally destroying the pure seed.

Then harvest the marked tomatoes separately when ripe, and process the seed. Fermentation is the preferred method. There are many existing threads & FAQ's on this, just search for "tomato fermentation", or refer to the link below which discusses many aspects of saving tomato seed.

Here is a link that might be useful: Saving tomato seeds