As said by many above, you can set and follow your own guidelines. There is no right or wrong. However, try to stick to your guidelines.
I go with "I'll take it" trumps ongoing correspondence. At first this may sound harsh, and somewhat unfair. However, I have found, this is the most fair. It is also (once again, I find) the only way to stay organized and sane if selling multiple cards.
Potential buyer #1:
Can I have scans/prices of A, B, C, D, E.
Potential buyer #2:
Can I have scans/prices of B, C, E, F, G
Potential buyer #3:
I'll take A, C, G.
Do I wait for #1 to respond to get back to #2, or 1 and 2 to get back to 3? I could, but what happens if #1 never gets back to me? #2 is on the fence ............etc., etc. etc.................etc., etc. etc.....
Although the grand majority of members do respond in some form or another. There are still some that do not* (or as stated by OP, back out). These are also reasons I go with "I'll take it."
* Not to tell anyone what to do: However, if someone sends you your request, please respond. A "thanks, but no thanks" is sufficient. Even if you think the seller is way off, a "you're nuts"
response is better than nothing.
//// end of babble