There is no reason for there to be a limit, although there are only a finite number of people reading the rec.toys.lego newsgroup.
If you're thinking of getting into speculation, make sure you understand the LEGO® product line and basic tenets of investment. Don't buy something for $20 and put it in a closet for 10 years -- it'll only go up to $200 by then, and you'll have kept it out of the hands of a deserving enthusiast for 10 years. Instead buy things that are already worth more than their sticker price and sell them immediately so you can reinvest the money now -- buy something today for $20 that you can sell for $40 today. If you do that several times a year you'll have *much* more than $200 in 10 years. Comb all the department stores and toy stores in your area -- especially Toys "R" Us and K-Mart. Look for things 3 or more years old.
The best LEGO® auctions are those that are a labor of love, and the best auctioneers are those need the money to support their own LEGO® habit.
"Drive-by auctions" occur from time to time and just don't have the same flavor. Watch out for people you've never heard of selling large boxes of LEGO® pieces -- not because they're dishonest but because they seldom know what they're doing, and if you don't ask enough questions, you may get hurt.
If you plan to hold a drive-by auction in rec.toys.lego, make your intentions clear -- don't say "I've got three grocery bags full of mostly Castle and Space pieces for sale, $200 o.b.o." and then sell it after only three replies -- wait at least a week, and let everyone have a fair chance to read your announcement and ask questions.
8 Jan 96 [TSL]