Purchases.com is probably at least $1,000 end-user, perhaps $200-300 reseller.
Purchasing.com is better, for sure. Purchaser.com is less desirable - it's hard to think of a commercial use for it. Purchase.com is excellent.
If I had to rank them, I'd rate them:-
1. Purchasing.com
2. Purchase.com and Purchases.com (different potential uses, both are nice)
3. Purchaser.com (a lot lower down)
On the other hand, many people don't like -ing domains much since they're longer, so Purchase.com might move up to
#1...