Keeping sales and purchase data in the same table? If my purchase and sales tables retain similar information, is there any problem with moving them to the same table and using an INT column to denote whether a row represents a purchase or a sale? 

If there's additional specific information for either, could I retain it in, say, a link table?
Even if you can at this point, it's not clear that you should.

We'll assume most of the potential technical issues don't apply: that sales or purchases don't connect to some other tables for example.

If you were going to do it, then shifting one or two columns, especially if not heavily used, off to a new linked table could work.

You could update indexes to include the new type column.

However, sales and purchases are logically different, and are used in different ways. Even if today they have few differences, that could change dramatically with time. New fields could be required, or new business rules or constraints, that might be difficult to support from the new single table.

Any reports would need to be updated, and would have to reference the new type column, and reference it correctly (which might seem easy, but....).

Various activities performed by your finance people could be complicated by this. If you ever moved to a double-entry accounting system, then the accounts affecting by the purchase of an item vs the sale of an item might be completely different. You would want them to sign off on the change.

Finally, you'd need to make sure that any auditors you may have to deal with would be OK with the change.

Meanwhile, what's the benefit? You might actually take up more database space (you have all the existing data, plus the need for at least one new column in every row). You've added another field to check to most queries involving the two tables now.

Ultimately, the two entities are logically significantly different, and your new structure hides that complexity, rather than emphasizing it.

All that said, I wouldn't say with 100% certainty that this change couldn't work out well for you - I would just want to be sure that it was very well thought out, that all relevant stakeholders were aware, and agreed - and, perhaps most importantly, that it resolved a real problem or concern about the current structure.