I did it a few years ago. When DS1 was 6, he wanted a lego themed birthday party. I ordered some bulk pre-owned legos with no problem. IIRC, I ordered a few boxes from different sellers. I had no problems with them being dirty or anything but I did check the feedback before I bought. We did get some unusual pieces but that was part of the fun of it.
My boys loved having a big bin of random legos to build with. They do have sets, but I think it helped them become more creative in their building vs. just following directions. I say go for it but watch feedback and shipping charges.
Karen
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Mom to 2 hockey-playing, Lego-loving boys DS1 2003 & DS2 2005