I remember asking when we were all talking about yard sales and someone (I thought a guru) had said just drag it to the street...but nothing (Not even yard sale table) will drag past the property lines. :/
ETA: I also built a flea market (had to set it as park and hide some objects below in non routable basement) and the Sims do pop into the community lot to buy once sale starts but those on already on lot don't bother just keep doing what they are doing. The 'start sale' option is there on community lot, but it just generates Sims to buy and leave. I guess that's o.k. but I wanted the Sims at the 'park' to buy, too. But money is money. :D