Is it ok to ask for SAE's as we do not get a lot of money and I can see postage being expensive if we have do send a lot out.
Yes, most people swap one kind of seed for another but if they want nothing it return it is commonplace for people to ask far an SAE or refund of postage costs particularly with bulkier items. Having said that some people do choose to cover the postage costs themselves. There is no fixed way of doing things.
It is also ok to say no when someone asks you for seed you are offering. If you feel uncomfortable giving somebody personal details such as your name & address (eg: you don't know them well enough) so you can exchange seeds then remember you are not obliged to do so, just send them a PM saying saying something along the lines of you would rather get to know them better on the forum first. People shouldn't be offended by such a message.
The thing that is not ok is selling seed or plants (unless done via the E-Buygumbay section) as that would breach the forum rules.
If someone regularly asks for seed but never offers anything in return and/or they never contribute to the forum in other ways they tend to become unpopular very quickly.