I have been questioned by the ATF about the number of firearms purchases I made over the course of 2006 and 2007. They did an audit of the FFL that I used and my name came up. Apparently they ran my name through an employment history database and I didn't show up as being employed. They then came up with a list of the firearms I purchased from other dealers as well. They showed up to my house and left their business card on the doorstep. I immediately hired an attorney, fearing the worst. This action proved unfounded, but I do not regret it. For those of you in Houston, I highly reccomend Sean Cody. He is an attorney and Class III machine gun dealer. He is very knowledgeable about firearms law and FFL procedures.
After a couple of weeks of being very nervous, I met with them and furnished them a couple of pay stubs and showed them the disposition of all of my firearms and brought the ones I still had. I would not characterize the experience as being "hassled", but I was on edge for a little while before the meeting. The agents I dealt with were very professional, and once it was clear that I had a job, they closed the matter.
There is nothing illegal about purchasing multiple firearms, so long as you are not selling them as part of a profit making business.