The only "easy" way to gain bells is to catch a restock of collectibles and resell it for profit. But restocks are random hence the quotation on "easy". Other than that, you'll have to put in time and effort through ways that Aleshapie posted above.
Posting may be the easier way especially if you don't have that much free time. It's slow but it'll build over time.
By far the slowest way, as Sholee stated, is also the easiest. Posting in a place other than The Basement board will give you TBT. The best way to get it fast is to post long and detailed posts as the longer your posts are, the more bells you'll get.