In my opinion, if someone sends you a ton of seeds, say fifty or so of one variety , surely they don't anticipate you using that many, so may want you to use those extras towards other trades, but I have also found on other sites this is offensive to certain traders, personally it doesn't offend me, and when others are trading seeds from their trades, the only thing I would expect is they let the other recipient know, they are not your seeds, and came from a trade. Since some traders to keep close records of traded seeds, you also don't want to be blamed for mislabeling, poor germ. Etc., so my advice to you would be when adding traded seeds to your available lists be sure to note that they came from a trade, also most traders will say in a note or private discussion about the trade that here are some for you and some to trade! At least in my experience, now as far as number of seeds, that is usually discussed in detail once trade is established BEFORE sending it, let them know how many you have and they will either accept or deny the trade, but really who needs fifty tomato seeds, or even 20 tree seeds, so be open about the number, general rule of thumb for me, tomatoes and veggie....I usually do 10 unless I have more than enough of them, flowers are approx. 25, most commercial packed seeds are coming with this amount as well ( prices are high seeds are low) if they a itty bitty seeds, then I would measure one 1/2 tsp or something larger seeds like peonies and camelias are a smaller trade around five or six, it really all depends on what you have , rare items will usually be smaller as well as they are harder to come by. Hope this helped....

I try to send 6 of the larger seeds and a dozen - 20 of the smaller seeds. If I can't see them, I don't count them - it usually comes out even in the end.

Many people list on their page, for example:
purpurea echinacea (C) commercial (purchased)
purpurea echinacea (T) from trade
purpurea echinacea (G) from own garden

If you save seeds from your own plant you purchased somewhere and it was a hybrid (as many are) be sure to add that information as the seed may not grow true. It may take on the qualities of the parent plant(s).
'Katie Saul' echinacea (G) *hybrid plant so may not grow true

I agree with what's been said. That is one point this website would do well to address, a Basic Etiquette and Expectations Page.
In the end it's better to ask more questions, give more information. If you only have 6 seeds, tell the trader. A few people intimidated me when I started because they asked me specific questions. It taught me a lot and helped me to pay a lot more attention. Now that I've traded some I understand why they ask. When I am interested in a particular variety it is irritating to receive an envelope labeled 'seeds'. Example: Rudbeckia or Brown-Eyed Susans. There are 1,000 kinds and many just label it Brown Eyed Susan, no more information. That's an Ebay label, but this website wants more detail.
Don't be intimidated or give up. I'm still a newbie and learning, but I love the learning part, too.
Heidi

I prefer to answer their email before sending seeds as sometimes I or they have more requests or offers. Makes it better when both parties understand what is being exchanged or just sent one way or the other. There are times that in the original request a misunderstanding was make and the correct information can be exchanged per emails. I will say that for me I have almost always gotten more than I have requested as well as been able to send at times more than the first request.

Thank you so much for the advice all of you have provided so far, this is exactly the kind of information I was asking for.