I let it depend a bit on what I 'need'. A lot of my stuff is given away for free, because it's in the way and I feel less guilty accepting other free things from people here.

I'm in a 'luxury' position of having enough bells and nook miles in the game to not really need more, anyways.
I've also traded DIYS for wishlist items or catalog trades, either setting up my own trading post or looking at those of others. Looking for other people that are looking for the specific DIYs (or any kind of item, really) I have is fun, especially if I know I can give them away for free. If I take IGB or NMT, I usually take 10-20K IGB or 1 NMT for a DIY. I personally would never offer things like 20 tbt for a single DIY, but I've seen people ask that much and get it. The most expensive DIYs I 'bought' were (I think) 1 bag of 99K IGB, 2 NMT or 5TBT, and I only bought those because I really, really wanted them.
I really don't mind 'buying' things, as long as I know what I'm supposed to offer. If you have a bridge to built and want me to donate to that, or want wishlist items, or are looking for igb, tbt or nmt, that's all fine with me, as long as I know in what direction I should make my offer. I think the most important thing is to make up your mind beforehand on an absolute minimum you want, and if there is one, disclose it. That way people will know what you're looking for, and can make offers accordingly.